RE: Fins s-type 137 2007
Good choices, you will likely find the Drake Pro more workable with big/ger sails in marginal conditions, sizes around 48 as you suggested but as the wind picks up going to a slightly smaller SR6B (~44) will be good with overpower 8.5 and mid range 7.5m. Both these fins are more efficent fins and are more suited to outright blasting than the Freeride Drive II (stock, which is compromised somewhat in outright performance - especially in marginal conditions in favor of versatility).
Cheers ~ Ian
Of course, there are a bunch of other options out there too, but from the Drake range these are the ones.