RE: isonic 111 - trouble with back foot
Just to get a proper perspective on things, I think that one needs to decide whether they are trying to handle the biggest sail in heavier winds (for high speeds), or using a bigger sail to get the most out of the lightest of winds. Although there may not be that much difference in the size of the sail used, the size of the fin can be quite a bit different.
I'm not a big or heavy person, but for a number of years I was successfully using a 50cm Tectonics Mirage on an older course slalom (284cm x 61cm x 115 liter) with an 8.1-8.3 sail in light winds (12-18mph average) with no problem. Really, I don't find it surprising that bigger heavyweight guys using 36-38cm fins on 7.8-8.4 sails might find that they have real no low end and struggle with spinouts.
If one still wants to keep fin size small for a fairly broad range, I would recommend checking out Wolfgang Lessacher's 100% carbon asymmetricals. His smaller fins have this incredible resistance to spinout, particularly his weedfins. I can't really explain it, but tracking ability of his fins are simply amazing, especially pushing hard to windward.