I have always wanted to try this, but i don't have the money for a U87. with the natural and linear response curve I would imagine, that using a U87 would yield very natural and warm tone but would retain all the aggressiveness that the cab has. In a sense it would accurately capture all the mojo of a cabinet as if you were there in person.
I have 2 main mics, a SM57 and a KSM27 (the condenser versions of the SM models) and found the 27 to be more natural warmer sounding. It captured the aggressiveness that the 57 did, but wasn't as harsh on the highs and was way warmer. A better more refined 57 basically with a flatter response curve. I could only imagine that the 87 would be a huge upgrade yet again.
but then again i like guitars to be recorded with condensers over dynamics, just my preference.