Sage
First off; you're gonna love your Surfyak. What I notice about mine, without the fins it was way too much fun, the only drawbacks I found were that when the wave got really steep it was difficult to hold a high line on the wave and a hard turn from the bottom into the pocket would result in the tail sliding out.
I put twin fins 34" from the tips of the tail 2" in from the rail with 3/4" of toe in over 10". The fins are 2 1/2" high. Now I can hold a really high line in the pocket, snap hard turns high on the wall, I think they may slow the boat down a bit for pure speed, and you'll find this thing is blazingly fast, but the up side is you can go to places on the wave where there is much more speed to be had so it more than balances out. I cut my fins out of 6mm plywood and put a double layer of 4oz on each face, then I use some Liquid Nails and glue them in place, I then use one layer of 4oz to glass them in place. This allows me to play with fin placement without too much screwing around and I'm not locked into one fixed configuration, as I would be if I used fin boxes. I have found that if I hit something solid, say a big rock what happens is that the fin breaks loose without tearing out the bottom of the boat, and is generally left hanging by a flap of glass.
As the fall progresses and the waves gain more squirt I'm going to give a Quad setup a try. I'm told you can work the fins and edges for more speed out of your turns. Oh when you go to turn this thing use your edges, hard, if you try and float your turns you'll be amazed at how fast, and how hard you can window shade.
Bob