I have examined the two different reflectors (without dismantling the headlights) and can confirm differences in the reflector patterns. This past summer when George, Dave, and I met at Petit Jean Mt. I was able to visually compare an 04 with factory HID (GWL's beautiful LSE) side-by-side to my 04 with halogen reflectors. I took some close up pics but there was a lot of glare so it was kinda hard to see. I can post them if desired.
Basically what it comes down to is that the HID housings are designed around the "bulb", as are the halogens. But the light is cast differently, then reflected differently to produce similar illumination on the road (focus and distribution, not intensity).