Remember that if you sail a lot in a spot with mostly marginal conditions (95% of the planet) 3 knots difference in planing wind IS A LOT.... It may reduce your planing time on the water with 50%..
To me, the switch from a 77 cm wide fast freeride board with 9.5 sail to a formula with 10.5 sail meant around 40% more planing time.....
If I was in the early planing iSonic mood I would go for the biggest possible iSonic and use a powerful 10+ freerace sail with it (twin cam deep belly). That would give good (almost formula) planing and excellent light wind speed...
Just a thought..