The piston doesn't affect the ability to increase the velocity/air pressure. The piston only affects the safety pressure at which the air will vent if the pressure gets too high. The two springs behind the piston affect the ability to increase the velocity.
Are you using the stock velocity adjuster?
Air rushing down the barrel before properly resetting is usually caused by bolt stick issues. In a level 10, that is caused by having a carrier size that is too tight. Tune your level 10 and use the largest carrier size that doesn't cause any leaking while sitting at rest.
Except for the Automag in front, its usually the man behind the equipment that counts.