I'm interested in this comparison as well, although actually don't need it anymore; the reviews at hand have been positive enough (Jude's review alone would have been enough) to convince me of the HD 800's outstanding qualities.
I still like my K 701, and it gets the most listening time of all my headphones. That said, I was always aware that it isn't perfect. Other headphones offer better performance in single criteria: my own electrostats in first place, HD 650/Zu, HE 90, K 1000...
However, I wish there was a headphone that ideally combines the sonic traits and virtues of K 701, K 1000, HD 650, ER-4, Audéo and my electrostats. And from what I gather, the HD 800 could be passably close to this sonic ideal. Neutral without being analytical, resolving and detailed yet smooth and musical, excellent 3-dimensionality... I'm looking forward to it or its European appearance, resp.
.