I hope you have a smaller board or 2 to cover about 18 knots and up.....
The Isonic 150 in 20-28 knots of wind would be very much like a formula
board and probably quite a handful to sail.
The extreme range I suggested was to help you get the iS 150 going in < 12 knots of wind (fin range in the 65-70 cm).
With a huge fin, and really good pumping skills you can often lower the min. planing threshold a couple of knots windspeed, but it's alot of work, and the board is not real fast on the top end due to far too much drag from the fin.
But...... you are planing when other sailors your size are not!
For normal slalom type sailing the 56 cm stock fin would be OK for the mid range (say 12 knots to 16 knots with your 9.5).... I'm guessing you switch down to your 7.8 m2 race sail in around 17 knots windspeed....right?
So, perhaps a good aftermarket 58-62 cm for normal early planing, a 54-58 cm (or use the stock 56 cm) for the mid range, and a smaller 48-52 cm for higher winds and your smaller sail.
I know fins cost alot, but you never have enough of them.
Sailing a board/fin/sail combination that are perfectly balanced for the conditions is like a dream. Doesn't happen all that often, but when it does everything works; your speeds are very high; and you can't miss a jibe if you try.
hope this helps,