Regarding observation that the CS boats did better than the SPs - It reminded me of what Ren Tolman said about his boats - "Build a Better Boat than you can buy!"?
Having the graphite powder in epoxy gives great UV protection and I think provides a smooth low friction surface. I have a Devlin Cackler I built in 99 and the bottom looks almost as good as when launched. Used about once a week since beginning. And I have a pirogue stored upside down and the bow gets sun maybe 6 hours a day and looks fine.
Carlins look great! I'm confused with your previous post with what appears to be a drain going from the mast (or mast step maybe) through the hull. ??? Never seen this before. My $0.02: When people say a mast step needs to be drained, usually it just means into the bilge to prevent water sitting in the "wooden bowl" that the bottom of the mast is in.
I like the shopsmith brand. One thing to possibly watch out for that I have read about and may have experienced is with sandpaper that touts no clogging. Apparently the stuff/chemical on the non-clogging sandpaper can come off and if it is on your boat surface it might affect your next layer of epoxy - so washing after sanding seems good practice as in getting rid of amine blush.