Without even looking at MESS, I can say that the MESS SNES emulation is pretty good but not up to the BSNES spec.
This is not an attack against MESS, mind you, but more of a note on how Byuu has been hammering on his setup for quite some time now. BSNES is likely to reign at the top for quite awhile at this point, as the others are far enough behind to represent a few years worth of work.
As for the MESS+BSNES question, that's easily answered. It wouldn't work that way. The underlying architecture of MESS is completely different. You'd have to completely rewrite BSNES to cram it into MESS, and if you're doing that, you're probably better off just doing your own code in the first place.