While I would still rate the iS50 as the ultimate iSonic for true speed use by big and bigger guys 25-30+ kts, I have to rate the iS76 as the most practical speed board for bigger guys wanting an "easy" ride to 40 kts on the speed strip.
There is absolutely no doubt that this new board has a lot of potential to be very fast, and inherently any "oversize" issue (c/w smaller pure speed boards in these conditions) is minimised by bigger guys who know how to put the power down hard...
A 2 size M-S combo to take a heavy rider all the way would be TR3/TR4 5.9 + 5.1m
On the other hand, if you're fully into 25-30+ winds and going for 40 kts board speed, a single TR5.5m would be a great one size option, and fully capable to take iS76 to 40 kts and beyond. The board and sail/s in these sizes for speed use would be remarkably rangy - and use of "minimal" equipment is really practical for some great results.
Obviously tuning fins down accordingly is also a good move; stock 32cm is above "par" for production, but obviously for pure broad reaching / Vmax fin sizes around 26-28 cm (2cm less if heavily raked) are a more ideal option for pure hi speed runs 40+.
Cheers ~ Ian