It seems a good stop (plus a little bit) faster for me. I ran some tests the other year with fresh Tri-X, T-Max 400, and T-Max 3200 developed in XTOL 1:1 and printed at the same grade in the darkroom (and scanned, but that's a different matter). Tri-X at 400 and TMY at 400 showed pretty much identical shadow detail for me, and TMZ shot at 800 and developed for 800 showed a tiny bit more shadow detail. To be fair, the TMZ came out with a bit more contrast than the other two with the development times, but nothing that harmed the prints or couldn't be accounted for with a tiny adjustment in the grade.
I also found that for my simple test, TMZ resolved visibly more detail than Tri-X.
I really like TMZ. For all those people who lament the loss of the 'old' Tri-X and how it's too fine grained now, think of TMZ as a one stop faster version of the old Tri-X. It can be gritty or surprisingly smooth, particularly when rated at 800. So many people seem to slam it because they've only ever tried it rated at 3200 in crappy lighting, and then proceed to underexpose it by another stop or two. In good light given proper exposure for the speed you are rating it at, it can be really nice:
The results of the test I mentioned above are here: http://125px.com/articles/photography/film/txtmytmz/
They might not be the most scientific, but it was semi controlled.