The MT-G2 LED wants a lot of current to really shine. Without changing the driver, you won't get much more heat (Or output) than the current LED. Upgrading the driver will cause a heat load problem.
Streamlights "C4 LED" means "Whatever LED we chose to put in." Lately it's been Cree LEDs, usually an XP-E or XP-G. Sometimes an XM-L. If the existing driver won't run well at 6v output, it will not work at all.