With all due respect, reading you post it is clear that you are way too obsessed with the slot, don''t understand the concept of powering up or down a sail plan, and really don''t seem to understand the role of the traveler in powering up or down the sail plan.
While closing down the slot too far can in fact slow a boat down, the slot has nothing to do with depowering the rig, as least as the term ''depowering'' is used in sail trim terminology.
In a general sense, there are a lot of factors in powering up or down a sail but the two biggies are the depth of its camber and the angle of attack. The deeper the camber (curvature of the sail) the more drive the sail develops, but along with drive (which is the forward component of the force generated) with increased camber comes a higher side force as well. This sideforce causes heeling and leeway. In really light air, the air flow lacks sufficient energy to flow around a sail with too deep a camber. As the wind picks up, you can introduce deeper camber which is how you ''power up'' a sail. Powering up involves and easing of halyards, outhauls,and backstays.
As the wind builds, so does drive but at some point hydrodynamic drag becomes the limit on speed, and at this point additional drive is not necessary. As this point is approached heeling becomes excessive. As the boat approaches this point the sails need to be depowered. To depower halyards, outhauls, and backstays are tightened. This pulls fabric out of the body of the sail, flattening the sail. The sail produces less forward drive, but it also produces less side force.
The second aspect of this discussion is angle of attack. For any given wind and sailshape, at any point on the sail, there is a proper angle of attack. If the angle of attack is too flat, the sail luffs, and if the angle of attack is too steep, the sail generates less lift and more drag and greater sideforces causing more heeling and leeway.
Because of gradiant 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 traveller 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 insigificant, 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 traveller 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 traveller 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 traveller can permit while still generating the proper downward force, at that point the Vang takes over the main role in controling 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 tendancy to dismiss this as ''racer stuff'' but these kinds of subtle sail trim adjustments can make for a much more comfortable and controlable 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 traveller 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 ennormous gains in speed.