There is lots of theory/text that sail twist use to adapt sail angle of attack because in top part wind is blowing stronger,so top of sail "feel" more true wind so must have twist...

I find some theory ( Frank Bethwaite high performance sailing) that say this is only myth.

"The wind gradient is not significant for sailboats when the wind is over 6 knots (because a wind speed of 10 knots at the surface corresponds to 15 knots at 300 meters, so the change in speed is negligible over the height of a sailboat's mast). According to the same source, the wind increases steadily with height up to about 10 meters in 5 knot winds but less if there is less wind. That source states that in winds with average speeds of six knots or more, the change of speed with height is confined almost entirely to the one or two meters closest to the surface.[39] This is consistent with another source, which shows that the change in wind speed is very small for heights over 2 meters["


But it is very easy to find out.Just buy 4 anemometers and fixed to bare mast on 1m,2m,3m and 4m height.Go on sea and read the velocity,if velocity are the same then this theory fall down.

So I dont understand why so much converstion about this ,if it so easy to find out...

I think that anemometes will show approx same veloctiy from 1m to 4m...