RE: Which sail type for racing narrow longboards
I'm not really sure about changes in sheeting angle ("a sailor using flatter race sails has a smaller movement distance between full-on power and low power. I think a sailor who makes trimming mistakes with a flatter race sail experiences gustier conditions than a sailor making the same mistakes with a deeper draft, more recreational sail (freerace, freestyle)."), so I'll have to check this out a bit. Never really thought about this aspect.
But, I've always done a bit better on longboards with deeper draft sails. This I know from years of experience.
Deeper draft gives more power, but does introduce some "control issues" when the wind gets stronger. But most racing is not in really high winds (at least not in the USA besides the Gorge and Maui) so deeper draft sails with more low wind power are normally better for longboards as longboards don't "unwet" to the same degree as a formula race hull will.
Another way to look at this is that the Formula hull has been designed to sail the apparent wind, with the hull pretty much unwet except for a short section at the tail of the board.
So a Formula race sail must also be designed to sail on apparent wind to the greatest degree.
Longboards have more drag, and while apparent wind is obviously very important (at least increasing your apparent wind to the highest degree possible certainly is) the long board does not get the huge "boost" in apparent wind achievable with a formula hull when it full unwets onto a plane.
So, as in almost anything windsurfing, there are more aspects to understanding why things work and were the advantageous "balance points" are than a simple look at them presents.
It's always more complex than it seems as you follow each design criteria along different paths. Change the type of board, and you need something different in the optimum sail. Change the windspeed and sail size, and you need to re-evaluate a whole bunch of other factors to achieve the optimum.
Hope this helps,