@tomfuegue
Sorry for the big delay: I don't have time for a exhaustive review, but here's my impressions on the S-XFi Gamer (compared to the CM MH751) -- as I don't have testing equipment, this review is totally subjective (so, feel free to take what I have to say with a grain of salt).
Basically, what the S-XFi Gamer is doing, is a much cheaper way to do what the AKG N90Q already does: the N90Q has a "self-tuning mode", where hidden microphones in each earcup register a signal that is broadcast to both ears simultaneously, and the resulting microphone data is then used to create a custom DSP filter for your ears (the filter will naturally be different for each person, as everyone has a differently shaped ear canal).
What the S-XFi Gamer does w/its software, which
@Inle covered in more depth, is to use photos to "attempt" to do the same (create a custom DSP filter), but the issue is, that photo-based data will naturally have less contextual data than microphone-based data, due to the inherent limitations of a photo (for example, why people look several pound/kilos heavier in front of a camera, than in real life).
In my testing w/the S-XFi Gamer, this came out a number of times: I had a roommate take photos of my ears using the S-XFi Gamer app on my smartphone, uploaded said data, then used Creative's software with the photo data. The results were always a lot less pleasing to my ears, than the AKG N90Q (which, admittedly, is a very good headphone): even when I tinkered around with the EQ settings, or used some of Creative's own settings (which did improve things significantly), it was still not nearly enough.
Going down a "tier" in terms of sound quality, my Audeze Mobius also blew away the Creative S-XFi Gamer in sound quality, whether it was gaming, music, or movies/TV shows.
Going down another "tier" to the CM MH751, it's pretty close: the CM MH751 has a bit more bass & sub-bass, while the Creative S-XFi Gamer comes off as having more soundstage & echo. The CM MH751 also seems to be a bit better with detail. I definitely enjoyed listening to the CM MH751 more, overall (not by a lot, though).
Conculsion: I'd go for the CM MH751 for music/movies/TV shows, while the Creative S-XFi Gamer would be my pick for FPS gaming. The S-XFi Gamer also seems more sturdily built than the CM MH751.
Note: I do plan to have more photos taken of my ears using that Creative app on my smartphone, in hopes of getting more accuracy (and thus, better performance), since the S-XFi Gamer's custom filter seems to be heavily dependent on the accuracy of the photos.
Hope this helps!