You wouldn't get the full potential of the hyper-resolving capabilities of the HD800, and the HD800 could end up sounding a bit dry and clinical. You will gain better depth to head-stage. And then the treble might be difficult considering that you are already coming from Cardas cables with the HD600. Nope. It's not sounding good to me.
Then again, you make like it if you are the type of person who finds german toilets appealing.
Okay, thanks for the input.
Can't say I get the toilet reference though...