The St115 is not a wave board and the very forward position seems a little wavy. When I sail my ST115 with the straps in the inboard position (for fast manouvers) I put the rear footstrap all the way back and the front strap in the middle position. This releases the boards rocker a little and gives it plenty of speed and acceleration.
With the straps in the outside (and rear) position the board becomes a very fast and controlled freerider..
Try to experiment with the fins too. I use mine with 32 to 37 cm and even a 32 cm wave fin too. It makes a lot of difference.