I get the feeling that many, many cheap lights use lambertian reflector designs that were never updated from the old Luxeon days; indeed, most of the lights that give awful beams with Cree XR-Es improve tremendously with a lambertian LED such as a SSC P4 (and XP-G, presumably, but I never tried that).
In contrast, the stock reflector on my K-106 seems perfectly matched to the XR-E package, and is indeed one of the only two reflectors I've seen that is relatively free of Cree-ring crud (the other being the one on my NDI).
Given that, I wouldn't replace the stock emitter with anything other than another XR-E emitter, and certainly not with a lambertian one such as a XP-G, for fear of getting precisely the awful beams you're getting.
Note that this whole post is based on the idea that the XP-G is a lambertian LED; based on what I've read this seems the case, but I might be wrong. If I am, disregard everything I've said about the XP-G (but the statement about old reflectors and SSC P4s still holds).