I went with the 95mm on my build, and I added a Stage 1 cam and did some clean up work on the cylinder head ports. I went with the 95mm as my riding is mostly back roads in 3-4 gear, and at speeds around 45mph....and I wanted to be somewhat gentle on the clutch and belt drive. The engine runs really well, gets good mileage and has a noticeable power increase over stock. It ran fine with the stock carb and air cleaner, and a DYNA muffler.
According to this computer...the HP increase based on the stock 30 HP, stock 8.5/1 compression ration and increase to 10.5/1 compression is only 2 HP. I sure think it feels like a bigger increase.
http://www.bgsoflex.com/cgi-bin/crchange.cgi?hp=30&oldcr=8.5&newcr=10.5Going from 95mm to 96mm adds another couple of HP and some torque, and the change to the 97mm adds a couple more HP and more torque.
If you are going to make the change to a piston....you really should do the cam as well. It will bump the HP up considerably.
Next time...I most likely will try the 97mm just for kicks!