Hi Wouter,
It is true that no windsurf rig has a mast longer then 5 mtr then ?
No, Formula or race sails are set on 490cm or 580cm masts with a mast extension, varies according to sail size. Check out Neilpryde.com - sails - racing - RS6 for specs. They have a 12,5sq.m sail that has a 6,1m luff length and 2,93m boom, so that answers your other question. (It is also true that by far most windsurf rigs have sail area's less then 6 sq. mtr. ?) They make a 5.8 and a 6.2sq.m sail too, so if you want to limit the max. sail area to 6sq.m, you`d pick the 5.8 and the luff is 440cm, boom 187cm, so stipulating a max luff length of 450cm and max. boom length of 190cm should work - but check out Naish, Tushingham, Ezzy, North and Maui Sails websites and cross-check their specs for sails under 6sqm just to be sure they would all fit into your class rules, or you will limit buyers to one or two brands that do fit. I would do this for you but lack the time right now.
You have raised an important point - most windsurf sails other than race sails have very little draft and a lot of twist, which might make going upwind quite difficult on a cat, since they travel so slowly (JOKE!!!) They are mostly made for reaching which most windsurfers do most of the time, it`s only the race and Formula guys that actually try to go upwind.
Check out the twist in this pic of a Naish Stealth race/speed sail..
http://www.windsurfingafrica.org/Portals/7/Gallery/Album/159%5CDSCF0468.JPG
Cheers
Steve