No true closed headphone will truly match up with the sound of the HD800 (FR, Soundstage, Detail).
The XC has a much smaller soundstage, more bass and a darker signature overall. Comfort is given up as well. The HD800 is thin and bright in comparison going between them, but with the HD800 imaging, soundstage and perceived detail will be higher, also in tow with the more elevated treble response. Both are fantastic I'll give them that, I'd consider buying the XC after my audition if I needed a closed headphone. I felt at the time that it was the brightest of all the Audeze's, while also having quite a large robust bass, a bit above in weight to the X ~ impossible to tell how it stacked against the LCD2/3 without A/Bing . Can't talk about the leak at all personally, didn't test.
The Alpha dog while good IMO is not on par with the HD800 in many technicalities, speaking from owning both. Still it is an option to consider + I find no issue with it leaking sound for anyone in my place. I think it excells at similar genres for me as the HD800. Very neutral.
The T5P I have not heard.
TH900 leaks enough to hear it in a room with people.
Ultrasone Signature Pro's leak nothing practically. Probably the best top model I've heard in this regard. It was a while ago personally for me, but I had trouble adjusting to the S Logic in my audition.
Sorry to not be more helpful.
Edited by Roronoa - 12/18/13 at 3:45am