Seen some black guys shoot a lot (heck, I make a lot, but I don't shoot far). It's all in the diet, if you ask me. I eat very healthy and I have for most of my life(after we left my dad, at least). The middle-class white group (I don't know if that's most whites but maybe) tends to have a healthier diet too.
So yea, I'd agree that white guys shoot more often than black guys but I'd disagree that they cum more (though, a lot of black guys in porn make that really THICK white cum... ew. High natural Lecethin or lechetin in the diet?).