I'd like to try that Danny Ching Fin (FCS). I would assume it would track pretty well.
The big California Classic Fin would be good for fun small waves on your race board... Try 9" or 9"1/2. Even 10. 10" might feel too similar to the stock fin.
http://trueames.com/shop/california-classicThis would be ideal for those Low Tide paddles, and other shallow locations...
trueames.com/shop/shallow-water-touring-sup-finTry a few, enjoy them, get back to us! If you don't like a fin, sell it on eBay.
Several good things about True Ames Fins, including a good range and colour selection. Does colour make it go faster..?

In flat water, the biggest difference you will notice is tracking. You will be able to count strokes and judge which fin tracks better.