Well as I see it the sail type makes a difference too. The iSonic is supposed to use a full on slalom sail. Normally hard core race sails do not have so much bottom end grunt (fullness and sideways forces) as their size may indicate, compared to more "free" sails. The centre of effort (the place where the power is generated from) is very low and very much forward and all that extra canvas up there is hanging loose most of the time mostly ventilating the sail at high speed in powered conditions.
If you put a 7.8 freestyle sail with five battens and a high centre of effort (great for manouvres, early planing and slashing around) and a tight leech I don't think you will find any pleasure in using this size on the iSonic.