I believe it's not the best match to put a nocam sail on the iSonic (otherwise it would work well with an 8.5). I also think that Steve's recommendations are a bit on the small side (iS111 & Futura122), but it could work in a pinch if you want to get rid of your stype115. Since you are near 100kg, iS111 is a sinker for you and your 8.5. You don't want to overload any board with the absolute max sail it can carry, so you should look at iS122 or F133 size-wise.
As for comparison between iS and F (or any slalom vs freeride for that matter), I haven't sailed a Futura, but you can look here for some comments on the subject:
Edit: Per, I was posting at the same time obviously. Since you are using ST115 with a 6.5 sail (it can carry more), I also thought you could go slightly bigger than a F122 for your 8.5