Surefire makes some great lights, but their rigorous testing means that sometimes their LEDs are not the brightest on the market. Several companies make drop ins meant to replace the P60 size bulbs or LEDs that are in the 6P, G2, and many other lights, but ThruNite makes some of the best we have found for a really reasonable price. They have three versions, one that has three modes for 2 cell (battery) lights, one that has three modes for 1 cell lights, and one that has one mode for 2 cell lights. The three mode versions work in lights that came with only one mode from the manufacturer thanks to some advanced circuitry that switches modes when you turn the light off and on quickly.
The drop ins are great for upgrading your LED lights to the latest and greatest LED, but really shine (brilliant wording, I know) when they replace incandescent bulbs that like to eat batteries, burn out after a short use, and generally be a pain in the rear.
Watch the video below if you want to see how you can upgrade your sorry old light: