Sailed the F-type 158 for more than a year. I used mainly a Drake 70cm fin with my V8 10,6.
F-type 158 is not the earliest planing board in the world, but the bigger fin helps somewhat.
By the way, even in 8-10 m/s wind, I never experienced the 70 cmfin to big.
I believe the stock fin too small, no problems at all using a bigger fin.