I notice my brass gets shorter in my Marlin so I must be alright! I agree there is no reason to hot rod the guide gun. Let your shoulder be your guide. If they are pleasant to shoot you are ok. If you can only shoot 10 of them, back off the load.
I hit a Mule Deer with a 400 cast @ 1699 fps and it went down right there like a lightning bolt hit it. It did more damage than I wanted it to to the tenderloin so from then on, I don't elk hunt at the same time as deer. Believe me, you wont need to load hot.