There is a skeleton XBOX driver in MAME now, although it doesn't boot anything.
The closest you can currently to get an idea of what xbox emulation would be like in MAME is the Chihiro driver (Chihiro is an arcade Xbox) You can run the 'outr2' set in MAME to test that. I'll save you some time, it runs at 4-5% speed on a 4ghz i7 and shows some, mostly broken, visuals.
the video was recorded with -aviwrite so appears to run at full speed, but keep in mind that 3 minute video actually took several hours to record due to the actual speed it runs at. You can see some actual 3d at around the 1:40 mark. The emulation crashes MAME (MAME freezes completely and has to be killed) at the point where the video ends.