Unfortunately, I'd have to say that this argument could be equally effective to gun control advocates in swaying people toward their ideals. For instance, we say that 5 extra rounds doubles our chances of successful self defense, but someone like Piers Morgan may say that 5 less rounds would reduce victims of gun violence by 50%. This isn't to say that I don't agree with the OP's statistically sound basis for contradicting the round limitation argument, but I just wanted to point out that this could be swung both ways. I know that I would feel much safer with as many rounds as I could reasonably have in a magazine than naively hoping that criminals actually have only 10 rounds in their magazines.
Marine, NRA member, SAF Defender's Club member, and constitutionally protected keeper and bearer of firearms