RE: iSonic105 technical ride
I weight about 70kgs, and I tend to agree with PG's comments. Very high volume boards really won't buy you much unless you also focus on matching them with very large sails. Given a sail limit at 8.5, I think that I would limit the board size to the iS111, or maybe the iS122 tops. Personally, my biggest board is about 115-120 liters, and my biggest sail is 8.3. While I can't claim to plane off in 6-8 knots, I find no difficulty at 10 knots with reasonable glide through the lulls. I think that it's important to remember a change from the freeride Carve to a iSonic slalom will most likely give you both a higher speed potential, to include a quicker planing capability, with the quicker rockerline.
However, before you go too far down the road, I would give yourself a chance to experiment with the iS105. Although an 8.5 is a bit large, you might find that it will tolerate it well, especially if you match it up with an appropriate fin.