Heavy pistons need a lot of spring to accelerate up to speed to make the pressure. If the seal is a bit below par too, it's not a great combination.
So what you're saying is, make a lighter piston and put a better piston seal on it (as well as putting the spring back in) and it will be much nicer to shoot and probably make power?
Pete
Far too many rifles to list now, all mainly British but the odd pesky foreigner has snuck in