Originally Posted by PowerHouse

The only thing that has changed is the shape of the head. Everything else internally is the same. The P5 should work as good or better than the GRP. I found the P5 to be stronger on the bottom end but alot of that will depend on the pipe, tune, clutching, gearing of the vehicle and the conditions you are running it in. If you were happy with the GRP, you will be as happy or happier with the P5. If you need more power all around, you can always look to the FMS or a .28 to fill the gap.