Excellent Wouter! This thread is about to be hijacked..

Attached is a drawing from a book by David Houghton, Wind Strategy - Fernhurst, giving some values for wind shear. I found it most interesting to see how wind direction changes according to what type if wind you have. No wonder a oversheeted mainsail is slow.

One point about twist. While not overpowered and sailing in chop/waves, it's still faster with twist than without even if it by the theory should not be. One theory I heard about this was that as the boat pitches in the waves, the angle of attack changes. Makes sense to me..