RE: Earliest planing with 8.5 m2
It really depends on your weight. The heavier you are, the wider the board and the deeper the fin should be.
With my 65 kg, I can start and sustain planing with the same racing 8.2 sail as following :
11 knots of wind with Carve 111 and 40 cm fin
10 knots of wind with HS105 and 40 cm fin
10 knots of wind with iSonic 133 and 48 cm fin
10 knots of wind with Formula and 54 or 70 cm fin
So, it looks like a width of 77 cm is optimal for my weight and I don't really gain anything with something larger than that.