I haven't given the HD800 a shot, but I did have the HD650 with the T1.1 and T1.2. My DT990/600 are my reference point, along with my standard audio chain. The Sennheisers and Beyerdynamics are not even in the same ballpark, IMAO. The build quality on all of them are fantastic, but the sonic signatures are wildly different. The 650, even modded, has the "Sennheiser veil" when compared with anything by Beyerdynamic and I'm trying to sell them as fast as I can.
I wound up keeping the T1.1 out of a balance of sonics and finances. The T1 series, to my ears and gear, are pretty closely matched. I managed to get a pair of 1.1 for about $400, while the T1.2 cost north of $900. I've said elsewhere that the T1 is a grown-up DT990; all of the wild characteristics of the 990 are more refined in the T1. Now, the $100k question is, is the T1 worth the price differential? The answer is a qualified maybe.
For my gear and ears, the T1.1 is an excellent upgrade. The T1.2 just didn't have the price-performance ratio to justify the delta. I really wanted to like the T1.2; the headband oozed luxury and sounded fantastic, but I can't tell the difference between leather and pleather when it's sitting on my coiffe. The T1.1 wins on comfort/price.
The T1.2 sounded a tad more refined on the highs, though neither was what I would call harsh. Even my most annoying Tori Amos tracks were smooth with these cans. The bass was tight and punchy on both. Only on "The Greatest Show on Earth" on Nightwish's "Endless Forms Most Beautiful" CD did the bass resolution really separate the two, with the T1.2 reaching further down the register and with more detail. This is not to say that the 1.1 didn't acquit themselves, though.
Again, I found the biggest difference between the T1.1 and the T1.2 was the price. Was the T1.1 worth the $400 ($150 more than the DT990) I paid? Yes. Was the T1.2 worth nearly $1,000 (four times the cost of DT990)? No. The T1.2 is a great set of cans, make no mistake, but the price point is misplaced. Of course, my audio chain isn't top of the line, but I really enjoy what I have; Modi 2 Uber, Crack, T1.1. The M2U was $100, T1.1 $400, Crack (after ridiculous mods) $750. For $1250 I have what I consider to be an outstanding binaural set. I don't see myself upgrading anytime soon.