I haven't done it, but I did go FROM a 2005 GTO (6 speed, 400HP) to the E39 M5 (6 speed, 400HP). Now before you call apples/oranges here me out. After having owned the GTO for a year I decided that one of the things I didn't like about it was the fact that it was a 2 door. Even putting my briefcase in the back seat was a bit more hassle every day with the coupe (as was picking up dry cleaning , etc. - much less having another person back there). When it comes to people, ingress and egress were the issue - everyone was fine once back there. All of this might have been more worth it if I felt like I had a nimble, tossable coupe, however, GTO (and 645i) is a HEAVY car. Ultimately, one reason I sold the GTO and got the M5 was because I figured if I was going to have a fat pig of a luxury sports car it might as well be practical. For my next car, I'll either be something similar OR I'll shrink down to a smaller, lighter car - the 1 series comes to mind.
Just my .02
