On a boat like the Hunter 25.5, as you approach a heel angle in the 20 degree range you are beginning to lose speed and ease of handling. The following is a piece that I wrote for another venue but which helps explain sail trim and shifing gears in a fairly comprehensive way.
"To some extent this is a question about adjusting sail trim to changing conditions, some times referred to as shifting gears. I will touch on some basics of sail trim. Most sails, but especially coastal cruising sails are optimized for a narrow range of conditions. Without changing halyard tension, sheet tension, or sheet lead, a sail is only at its best in a wind range that is often well less than 5 knots in width (i.e. 7 to 12 knots, or 5 to 10 knots). By making adjustments to the 'flying' shape of the sail you can easily extend this range by 5 or more knots either side of this ideal setting. These changes can include 'powering up' or 'depowering'. In sail trimming terminology these terms have very specific meanings. They do not mean reducing sail area.
In sail trimming terms, power is the force (lift) generated by the wind passing over the sail. This force is thought of as having two components. One component is seen as being perpendicular to the length of the boat. This component causes heeling and leeway. The second component of this force is drive, a force considered to be parallel with the direction that the boat is going.
If you make a sail fuller (meaning more rounded and in sail trimming terms 'powered up') you generally increase the force generated by the sail. This means an increase in drive but it also means an increase in that side component that causes heeling and leeway. Conversely, if you flatten a sail, you produce less force, both drive and side load.
Beyond power, there is second and very important aspect of sail trim, which is incident angle. The angle of the sail to the wind is referred to as the 'incident angle' or ‘angle of attack’. To give an example how angle of attack works, if you think of what it is like to drive down the road with your arm out the window, if you hold your hand flat, you feel a small force pushing your hand back away from the wind. That force to the rear is drag. If you turned your hand so the front of your hand pointed slightly upward, you felt a little increase in pressure rearward(drag) and your hand would be pulled upward (lift). As you increased the angle of your hand, lift would increase but so would drag. At some point of rotation, as you hand approaches vertical, the amount of lift begins to decrease and the amount of drag increases dramatically.
I want to clarify two more terms for our discussion, luffing and stalling. Anyone who sails understands luffing but those who are not into sail trim terms will often use stalling and luffing interchangeably. They are two very different terms but both refer to a particular type of incident angle. Luffing is an incident angle that is too flat to the wind to generate lift (like your hand in the flat position). Stalling is a bit more complicated to explain but it is that condition where the incident angle to the wind is too steep. To generate lift efficiently, the air needs to be able to flow aft on both sides of the sail, ideally all the way aft to the leech of the sail. When the angle of attack gets too steep the air can’t make the necessary sharp turn aft. The leeward side of the sail is a low-pressure zone and the air that fails to make the turn is sucked backward into this low pressure, tumbling turbulently as it does. The failure to maintain flow is stalling. In the example of having your hand out the window of a car, it is analogous to having your hand at too steep and angle to the wind, which is increases drag and decreases lift.
So, back to the questions at hand. As the wind increases in speed, the sails generate more lift, both components drive and side force. This happens due to a number reasons. First and most basic is that lift is roughly proportionate to wind speed. So as wind speeds increase the amount of lift increases. There comes a point with displacement boats where almost no matter how much additional drive you add, the boat will not make greater speed. At this point the lift force of the sail can only go into heeling and leeway.
But besides the simple effect of increased wind speed, the increased force on the sail will cause them to power up on their own. This occurs in a number of ways. Because sail cloth can stretch, as the sail stretches more length of fabric is added and so adding more curvature is created and with that more lift is added. But control lines, (halyards, sheets, and outhauls) stretch as well further allowing more curvature in the sail and further powering up the sail. Stays and shrouds stretch as well and they further add to the fullness of the sail. So as you enter a gust, you sails are automatically powering up (becoming fuller) just when you want flatter sails to reduce heel, you have fuller sails increasing heel.
Because of gradient wind effect, (slower air near the water than higher in the air due to the friction between the water surface and the air above) in light air, the apparent wind angle felt by the sail will be different at the head of the sail than at the foot. The apparent wind at the foot of the sail, will appear to be more forward than the air at the masthead. To allow the sail to have a proper angle of attack twist is introduced into the sail so that the upper part of the sail has a different angle of attack than the bottom of the sail. Here is where the traveler, backstay, and the boom vang come into play.
By bringing the traveler to windward, the pull of the mainsheet becomes more horizontal than vertical. In doing so, the boom is held inward toward the centerline, but the boom is allowed to lift a little, and that lifting eases the tension on the leech of the sail allowing more twist to develop.
As the wind builds, gradient effect generally becomes insignificant, so the whole leading edge of the sail wants the same angle of attack and in general, that angle of attack needs to be much flatter than it would be in moderate winds. To unify the angle of attack, the traveler is lowered to leeward and the mainsheet tightened, which increases the downward force on the leech of the sail. This increased leech tension removes the twist from the sail. As the wind builds the angle of attack can further be lessened by lowering traveler further to leeward. As you bear off on a reach, the traveler can be further lowered to maintain the proper angle of attack without powering up the sail, but at some point the sail needs to be eased broader off than the length of the traveler can permit while still generating the proper downward force, at that point the Vang takes over the main role in controlling twist and the sheet then simply controls the overall angle of attack of the sail.
Simply easing the mainsheet in a strong breeze does allow the head of the sail to twist off and reduce heel, but it comes at a price. In easing the sheet the boom rises and allows more fabric into the body of the sail increasing power just when you need to reduce power, and also in order to obtain enough drive, the lower portion of the sail is overtrimmed developing a lot more weather helm than would occur with proper sail bladed out sail trim.
The backstay tension (especially on a fractional rig) can be used to depower the rig further. On any rig, even one with a stiff mast, tensioning the backstay removes sag from the forestay and is doing so, draws fabric out of the sail in a horizontal direction, flattening the jib and depowering it. As the forestay is tightened the mast moves aft and that also changes the relationship between the jibsheet lead and the head of the sail, allowing the leech of the jib to open slightly, reducing the angle of attack of the upper portion of the sail. On a boat with a bendy rig, and more dramatically and controllably on a fractionally rigged boat, as backstay tension increases the mast bows forward, in doing so it also draws fabric out of the sail depowering the sail in the same manner that tightening the forestay flattens and depowers the jib. Also similar to the jib, the masthead moves aft as the backstay is tightened and that opens the leech slightly at the head of the sail, easing the angle of attack and further reducing heeling, weather helm and leeway.
There is a tendency to dismiss this as 'racer stuff' but these kinds of subtle sail trim adjustments can make for a much more comfortable and controllable passage as well as adding significantly to the speed of the boat.
Lastly, really disagree with the idea that "if your sails are basically trimmed correctly, and you are making the kind of fine adjustments that a skilled racer would use, those fine adjustments can't be measured by your knotmeter". Small adjustments to backstay or traveler positions can tremendously reduce weather helm and heel angles. On my prior 28 footer, these fine tuning items were good for a half-knot or more, and on bigger boats or in higher winds, these kinds of minor adjustments can yield enormous gains in speed."