The S-25 comes with three head shims, you can take up to 2 of them with anything hitting. This will decrease head clearance, thus increasing compression, thus increasing horsepower.
I read this in RCCA and tried it and it really added quite a bit of power. You probably shouldn't run 30% nitro if you take out any of the shims.
A new pipe is also a good idea and I hear good this about the THS set.