Hi JT, not the SB team not as your on Selects I thought I'd chip in
Since you bought the S10 last year, Select have brought out the new S11 for 2011, the S10 is still in range as well. The S11 is more power ride IE S11 47cm will have more power than S10 47cm.
For a stock fin, Drake R13 52cm was pretty good, but a Select S11 53cm will give you much more power on tap. The iS144 has a OFO 57cm so the max size fin for super light days (12k and lower) would be 55cm. The 55cm will become a handfull if the wind picks up, but you can change down fins.
If you can find a used Select RS7 53cm or 55cm, it will be very good as well. The RS7 is the same shape as the newer S11 but slightly different carbon lay up. Same power, similar feel but half the price
I'm guessing that on super light wind days you already have your mast track right back to get on the fin, and your already tuning the sail for max power.
Just my 2 cents worth.