Well we're not racing Hobie 16's here, but modern boat with real trim controls,

Oversheeting the modern mains can in some conditions kill upwind speed.

Pretty much you always went the squaretop to twist off a little bit. if it is in too far then you are just creating alot of tip vortex drag.

Pretty much you increase downhaul together with mainsheet. This keeps the twist profile the same while flattening the whole sail. This is fast in the right conditions.

Go over to the F16 forum and do a search on trimming and tuning the sails. Over the years we have covered this topics plenty of times. Spitfire sail trim should be very comparable to F16 sail trim.

Wouter