You are over hammered for a medium reg setting.
For 135Bar the factory 30Cal hammer weight and spring is plenty. The heavier aftermarket hammer weights and springs are meant for high reg settings. You will likely need to increase your reg pressures, but FX recommends not going over 170Bar on AMP regs. What size barrel do you have? The shorter barrels really give up a lot of velocity no matter what you do. In my Wildcat Compact at 140-145Bar with the 13gram hammer and correct 52mm spring I was only able to get about 70fpe with Nielsen's lightest 30Cal slug...43.8g?
EDIT: Sorry I forgot the Wildcat/Maverick Slug Kit hammer weight is the same 13gram the 30Cal should have installed from the factory. But the rest of my above thoughts should still steer you in a direction, maybe even the right one haha.