RE: What is your fastest 2 Sec GPS with an 8.5?
The best so far I did with HS105 & RS2 8.2 sail was 30.2 knots Vmax in 12-15 knots of wind on flat water. To do so, the key ingredient is fin size, type and stiffness between either a Deboichet speed-slalom 32 cm medium stiffness or a Select Elite Slam S03 32 cm hard stiffness. The Select is much more spinout resistant than the Deboichet in overpowered +15 knots of wind with my 65 kg as I can crank as hard as I want without any problems.
So, if you want to increase your Vmax, I suggest you start to use smaller slalom fins, especially on HS. Check that old thread between race vs slalom fin's choice for HS : http://2006.star-board.com/forum/askteam/read.asp?ID=4615