This is what kids had to say about the Prokids Sonic (i.e., the ancestor of the current iSonic50) :
This is gonna be very tricky.
On one hand for long distance racing, beware of the low volume of the iSonic 50 : all the kids said the board needs wind and you might loose the planing sooner if you hit a lull ! You might be better served with the iSonic 76.
On the other hand, if sailing with a 4.0 sail in high wind, you will have an easier time with the iSonic 50 board which is perfectly tailored for kids of your size.
Any chance you might use a sail smaller than 4.0 in strong wind ?