Although I can't specifically comment on the iS76 and your Sonic52 from experience, I thought I could offer some support for the newer wider designs. I was in a position of updating my 1999 ML 8'10" slalom board (which is still an absolute pleasure to sail) with one of Mike's latest designs. After receiving the new board, I took the opportunity compare the two boards side by side. The differences were clearly night and day. Although the new board was only about 3cms wider at the wide point, it was tremendously wider in the tail. This huge difference really stood out, and I wondered whether the new board would have rough water control and ease of turning I really enjoyed in the old board. Another two things that also stood out were the obvious differences in length (8'10" vs. 8'2"), and the fact that the new board had a much stubbier fuller nose shape.
Well, I can say my worries about whether the new board could deliver the goods were erased completely once I had the opportunity to run it through the paces. However, as Ian so aptly noted in his post above, the newer wider styled boards do take some adjustment in your stance and technique. I've found that the new board is arguably faster, offers great control in heavy chop, and it jibes significantly better. One wonders how can this be true, given the sleek racier shape of the older design. Well, it just goes to show that some of our well established paradigms can sometimes hold us back from taking a different, often unclear path.
While you might prefer to hold back on the iS76 until you have an opportunity to take a test drive, I'm sure that you'll find like I did that all that width really does work.