Having said that an averagely well designed boat can hold full-sail in 25Knots, gutsing 30/25, when a crew is available for good trimming through the gusts. Short-handed, or a skipper with an inexperienced crew would probably do well to reef earlier.
That depends a lot on the type of boat and the size of the head sail. You are probably talking about an old boat or an heavy/medium displacement boat. Modern boats, depending on the design normally will have to take the first reef sooner, between 18/22K wind and the second between 25/30K. Normally with automatic reefs boats will not have a third reef but if you mount one it will be used over 35/40K wind, this off course assuming going upwind.
Up to around 30/35 knots gusting, if the skipper is competent and knows his boat, then roller reefing 'is' an option. But remember, your full headsail is a light-weight sail designed for modest conditions.
Further, if your boat is a bit tender then as you roller-reef you step the centre of effort higher and higher, which can increase heel angle.
How you reef your boat, meaning the order when to roll the head sail or put a reef on the main and the balance between the two depends much on type of boat and the way it is rigged (big head sail, relatively small main or big main and smaller head sail). In some boats you can go faster having almost the full genoa with a reef on the main others will work better with a reefed genoa and full main and others, with a small head sail, can have it with a lot of wind and you just have to reef the main.
In truth, if you have roller reefing on a 120/130% headsail it would be better to buy a much smaller, much heavier haedsail with a wire luff, and fit a snap ring into your foredeck and use your spinnaker halyard as the hoist.
Make sure you buy such a sail where the clew is in the same line of attack as your main headsail's. That way you can simply unclip the big one and clip on the small. Furthermore, in doing this, the cenre of effort is kept as low as possible.
Yes I agree but what you are talking about is practically the same thing has having a removable stay sail. But guys take care, it is not just enough to fit a snap ring on the foredeck. The forces that are going to be made there are huge. You have to reinforce all the area and have a way of transmitting those efforts to the structure of the boat. It is better if it is provided by the manufacturer or made by someone that has the experience and knoweledge to do that