The HD800's do not perform like speakers when it comes to soundstage and imaging, not by a long shot. While their soundstage is a big improvement over their predecessor, the HD650, it's the not the best imaging in headphones I've heard. Very good, though. Definitely better than most headphones.
However, the tonality and frequency response are the closest to real life that I've heard in headphones. They would make perfect monitors for mastering classical music in that regard, but I'm not familiar with mastering classical music, if soundstage and imaging come into play for mastering recordings, the HD800 (or any headphone for that matter) would not be the optimal choice.
While headphones would be better suited to on the go and location monitoring, the HD800's open nature makes them a very poor choice for that type of monitoring. In the end, speakers are still necessary.