If you've seen some pictures of the Fenix and other similar lights, you'll notice that there is no easy way to remove the light engine. There are no drop-ins available and the R2 flux bin of the Cree XR-E LED is currently the highest flux bin available for their single die setup. There's nothing easy that you can do to make it brighter, as if what you have isn't very bright already.
Also the DX's "R2" drop-in probably doesn't use a real R2 flux bin and the drive current is pretty constant meaning that 6v and 9v shouldn't make much of a difference in output.