It's worth considering that one of the target markets for this size is the "light weight" slalom racer, and the iS76 has not been developed as only an extreme wind board for big or bigger riders.
You will find the new iS76 sails a little larger and requires a slightly different technique to the razor narrow Sonic52, however for both overall control and overall speed, you will find that the newer iS76 is a more versatile and less "peaky" design, which will be more significant in gusty conditions, where the "carry" (glide?) of the iS76 shines thru.
The iS76's "safer" mid and forward section rail shape gives a lot more confidence in chop, and really allows the rider to "press on" when they would previously be thinking very seriously about trimming or backing off !
Cheers ~ Ian