Here's the thread on the Uber Upgrade. I did the upgrade last night and the difference was immediately noticeable.
I cant specifically talk to the HD800... yet... but with the HE-500 the bass tightened up, better extension across the spectrum and smoother mids and highs with less grain. I'm suspecting that I'm getting more detail also but it's hard to tell. It seems a little brighter but that may be due to the upper end extension/detail. Need to spend some more time with it.
I didn't do the Gen2 USB upgrade. I do have both USB and optical hooked up with my MacMini but it's really hard to tell the difference. Maybe when I get the HD800...
Hope this helps, M
I did my before/after tests with the 24/96 Rumors album. The recording has to have detail to really appreciate the detail in the uber upgrade. Boy does that album/recording have it in spades. You're going to love the 800s.