For me the courseboard was easier and faster. But I also spend more hours on the courseboard and that makes a big influence as well. Since the board is bigger it is more stable and can gap chop better. If perfect condition I believe the slalomboard was faster but they are rare..
I used 32/34 tectonic talons on the slalomboard. The courseboard I used 37,5/34 HP fins for slalom and everyting from 38-42 depending on the wind for courserace.
I still have some Tectonics T43 40,5/38 and 39 with 2 degrees cant for sale if interest (300 € for a set).
After the Red Bull Ocean2Ocean (where I finished no. 8 ) I sold everything and started foiling.
Regarding how the difference in size influence the performance. In short: if fins are too big you are foiling too early and cant controle the board in speed. If fins are too small they wont give you enough lift to get you going. Thats why you need bigger fins in light wind and viseverce. Regarding cant. More cant=more lift. 1 cm in length is = ap.1 degree in cant.
The longer the fin the better upwind performance but harder to controle downwind. So it is about finding the best compromise between length and cant. Plus 1000 other factors