I suggest .6 because thats what we found worked for our racing team .. it lowers comp and helps combustion at high speed.. Its very hard to tell why your piston shattered . Anything could contribute to that ... if the piston was faulty it would break in the first couple of tanks normally .. having said that the Go Tech engine has to be one of the best quality checked engines on the market bar none.. so faulty parts is very very un likely .

What you have to consider is when a piston is fin full flight anything gets in its way and it can break no matter what the brand of engine.. So i would check for any foregn matter inside the engine .. possibly taken in from your air filter or in your fuel tank .. This can also happen when you change plugs .. Some people dont realise that there is dirt around the plug after running ... if you do not clean this off before changing it you risk dirt getting in through the re...

Hope this helps