I agree with Roger as I too have a iSonic 111 (wider tail than the futura) so it should plane earlier. I have used my 8.4 Maui Sails TR-3 race sail on the iS several times. I can't plan in 10 knots (small white caps beginning to form) and I weight 78 kg.
Our winds are highly variable, so on a 6 - 15 knot day, I usually don't try the iSonic since I won't stay on plane very much. I go for a formula board and an 11.0.
At your weight, the 8.5 and a steady 10-12 knots should get you going just fine, but it all depends on how steady your winds really are. Any holes under 10 knots and you will come off plan.