Check out the main T1 thread - they must have been compared to just about every other headphone going over the course of the thread If you search the thread for a specific phone like the HD800, you'll see a lot of impressions. It might be a bit skewed in favour of the T1 but I've actually read some pretty fair comparisons over the course of time.
The T1 has a different presentation to the HD800. I lived with both phones for a year. The T1 is faster, punchier in the bass and has more mid emphasis than the HD800s. The HD800s sound smoother and have a larger sound. I listen to Rock and Pop and some classical. I preferred the signature of the HD800s ultimately.
Your ears are the best judge.