The German Surf Magazin tested "light wind planing boards" in their latest issue: JP Super Lightwind, *board iSonic 150, Futura 155, Lorch Bird 149 and 179.
The conclusion was that the best board for effortless lightwind planing, in combination with spirited performance, was the Lorch Bird 149 at 74 cm wide and 261 long. The lesser width was compensated by a longer drawn out rocker line (compared to the 238 of the JP, and teh even shorter Starboards).
That is, the bird was more efficient than the JP at 90 cm width, and in particular than the iSonic 150 at 93 cm width! However, the iSonic was faster as it is a pure slalom board rather than an early planing machine (more suitable for those who like to pump).
Thus, weight is important for early planing, but length even more so...
If you still go down the iSonic route I think that you don't need to go any bigger than 121 at your weight.