Look at it as rate of fire. With 30 round mags its about 50 aimed rpm. With 10 round mags its more like 30rpm. He can kill people only 3/5 as fast. Ceteris paribus therefore he will have the opportunity to kill 3/5 as many.
That's the theoretically possible maximum rate of fire. Actual rate of fire appears to have been maybe 12 rpm with 30 round mags. Given 10 round mags, he empties a mag in 50 seconds, and takes 10 to reload. His rate of fire dropps from 12 rpm to 10rpm.