Are you the guy who bought my U853 caps recently? I don't remember who I sold them to. At any rate... I did that before I sold them to someone recently. I don't know if I still have the on my computer or not (I'm not home). What I did for a recording was hold them both about 3" from a boombox speaker... not really loud, but enough so they were both getting the same sample.
The result was this... My pair of U853 caps matched each other closely, my AT853 caps match each other closely, but the U853 caps and the AT853 caps didn't match each other in sensitivity... one was more sensitive than the other by quite a bit... like 6db or more, but I don't remember which was which. Once I did a "normalize both sides to -3db" in audacity, they didn't sound drastically different... If you are looking for how much "hiss" was in the background... I don't remember there being a difference.
I'll try to remember to look on my computer when I get home, but I think I just dumped them the other day when I was doing to some routine cleanup.