if it was me, and this is just my personal openion here, but in the windspeeds people are talking about here, i think your best bet is gona be an aussie 18 with two light friends ;p very tough to beat those boats in such litle wind, so this is what i do since i A) dont have two light friends that i would trust to sail an aussie 18 with me, B) dont have an aussie 18, and c) dont have anywhere near enough spare coins to fork over to buy one, this is what i do...
5-10knts (gusts of 15 max) A) sit at home and grumble that is there is no wind, or B) sail a 29er with my friend.
10-15knts (gusts of 20 max) A)7.2/6.6 (both race sails) on 115L slalom board or B) have ALLOT more fun with my friend on his 29er.
15-20knts (gusts of 25 max) 6.6/ maybe 5.8 (race and freeride) if the base is in the high teens on 88L slalom board, or B) may take the 29er out if we are feeling lucky;p
20-25knts (gusts of 30 max) 5.8/maybe 4.8 (freeride sails) on the small slalom board again.
25-30knts (gusts of 35 max) 4.8 (freeride sail) same board.
after that 3.5 up to gusts of about 45knts but that happens once in every two or three years where i live so not much practrice for that set up...
the norm around here for a windsurfing day is probobly the 15-20 max of 25 so u can imagine i gruble allot at home !!! haha
take into consideration that i could easily use much bigger sails if i had them and if i wanted to for the same wind speeds, but i find it kind of tasteless in my openion to use such big sails with the entire leach twisting when a smaller sail could be used almost as if not with the same succes (for me anyways with my barely 70 kgs) and have a much easier time uphaling or water starting if the wind is constantly over 15knts (still kind of an ameture ;p )
anyways, just my 2 cents and as BelSkorpio said, " Hey, we can not be all professionals, right ? "
fair winds all!