RE: isonic 111 vs sonic 110
While the basic dimensions are not too different, the way the board rides(trims) really differentiates the two designs, and it is fair to say that the "feel" is noticeably differnt in most cases.
I think that most guys who have made the switch are pretty happy with the result. It does take a couple of sessions to really get in the groove with getting the most from the iS but when you do , it's pretty hard to go back in that size range. Maintaining an even and very thrashable trim at speed, especially over moderate chop - and while handling overpower sail loads nicely- is probably the main positive attribute over the S110 etc.
The overall Vmax speed really won't be quantum faster on the iS111 than S110, but in most conditions iS will offer deliver measurable advantage - something that has not escaped notice on some major race results this year.
If you're running medium speed conditions (W Kirby) where the water is relatively flat on the down speed run, then this is one condition that you may not see such a Vmax advantage to iS until either the speeds go a bit higher (and the trim/lower lift front end of iS starts to advantage) or if the water starts to get a rolling chop into the speed run, in which case the "power on" trim of the iS will make it a bit more sailable and help speed potential. If you're blazing back upwind through short sharp chop, the locked down trim of the iS encourages a more "power on" style, which gives a better upwind speed potential, aided also by the wider tail/stance.
In either iS111 or S110, if you are really chasing top speed trial run speed (2sec etc) then you should consider less fin (for sure) than 40cm. With 40cm and 7.6, you're haulin' a stack of drag along, and although you might struggle a bit upwind on a smaller fin, I'd be looking at a good 34 (or similar) for hitting your best on that kit on speed runs.
Cheers ~ Ian