It didn't take too much effort, just a little time and patience. I used a scrolling saw to do the rough cut and then smoothed it out with a sanding drum on a Dremel tool. I left a bit of the lip on the tail end of the sponson, but you could easily cut that off entirely. Those with a belt sander could do a better job of smoothing out the cut, but I don't have one. Hopefully I'll get out on the water tomorrow to test them.
One of the pics is of the stock F-15X sponson right next to the stock F-12X sponson. It's pretty clear that the lack of a lip on the 12 allows the back end to break loose in a turn. Fingers crossed.