The knee-action toggle assembly of the Luger 'slaps' the rear wall of the grip frame on opposing sides; the breech-block strikes from inside and the rear toggle 'ducktail' from outside. Increasing the charge just makes the 'slap' harder.
I agree with Seager that for best results with these old technology pistols you should work up a load that just allows the action to cycle reliably. A heavier mainspring requires extra powder charge just to cycle the action. It also gives the 'slap' the effect of hammering the steel.
If you want higher velocities, use a lighter bullet.
...My $.02, and worth every penny you paid for it...
