I can't speak for every size of iSonic ...
But I do still have some of the older smaller boards from 2006/07 to compare to the current '12 lineup. I believe the newer boards do respond well with older style fins. ie, I still have some Deboichet fins that I picked up a good 6-8 years ago that suit perfectly the newer (and older) boards.
That being said, in the past 5 years we've had radical fin development improvements in slalom boards. I see a lot of the fins nowadays under the top guy's feet have taken a lot of the ideas we've discovered in formula fins and applied them to slalom: softer bend curves, more swept-back outlines, more twist... and I do believe these newer fins are a definite improvement vs older Deb/Select/Tectonic shapes from many years ago.
So I don't think it's the board's fault, I think we've just seen newer and improved fins on the market. The older fins still work perfectly now, but the newer fins can give you the edge, and I think they've played a roll in extending the windranges of the boards...
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