I forgot to mention:
I liked it the original setup with the stock fin with my 5.7...my first setup in the water when I tried it.
Also, for more speed and slalom keep your straps tight,....this opposed as if you prefer freestyling...
Finally, dont know what type of sail you are using, but with S-type I like medium size harness lines for freeride-slalom (not too short) and longer ones for freeride-freestyle. It feels a lot more confortable with this stance in this board.