RE: More iSonic Questions...
The above is what has worked for me.
If you really want fast, try shifting a little weight onto your front foot (this is Ian Fox's Isonic "turbo charging" tip) as you take some "push" off the fin.
It seems to "settle" the board a bit and cause you to bear off slightly, increasing your speed in the process.
As far as control in chop, a smaller fin can often make a world of difference.
A lot of guys are using < 30 cm vertical slalom fins (similar to the OEM Drake 42 and 34 cm's) with great results on the smaller Isonics.
I haven't sailed the Isonic 125, but I'm pretty sure that the design parameters are similar with the 115 and 105, so the tuning and techniques needed would be pretty much the same as well.
Don't be afraid to use a really small fin that seems to "spin out" and not give you good upwind at slower speeds.
When you lite the Isonic up, the water gets flowing faster past the fin, and a really tiny fin can feel "solid as a rock" once you get the flow needed.
Hope this helps,