I have a ,308 spring and heavy 7.5 oz buffer in mine currently. Shot some WWB and some freedom munition 115gr. recoil seemed a bit less than a friends PCC I had shot previously. dot was staying in the window for the most part. but would like a little less if I can. When I start actually loading 9mm ammo for this thing I will try the lighter combo. In my .223 the less reciprocating mass you had, the lighter the recoil felt and the less scope/sight movement I had. I can't see where this would be different.
If i go that route, ill keep the "heavy" setup in the bag for factory ammo, etc if needed.