A small percentage of extra performance.
I've been running a mildly modified M2 for around 6 years. It will run Fiocchi 1150 fps slugs and anything I've put through it. The bolt has not been modified (Kurt said light loads don't like light bolts), I polished the trigger some, beveled the loading port, 10-22 rear sight, deburred the extractor cut, bolt release button, and have a modified lifter. I modified the lifter I had welded to the way I liked it. I was told it is close to the old M1 lifter, it has a 1/4" radius cut in the front edge, rather than the long, bent version.