So I returned from my trip and took my friends 550xi home from the airport. On the downside, that glove box door is a piece of crap - can't believe they put a door like that in a $70K car. I also REALLY miss my leather dash and Nappa seats. My 650GC feels so much more luxurious. Oh and the exhaust - the 550xi does not compare to the 650GC in terms of exhaust note (I'm not even sure the 550 has an exhaust note). Oh and I hate the auto trunk - I'm always waiting for it - can't believe I'm saying it, but I like the manual operation of the 650 more.
However..
Man his 550xi feels FAST and nimble. He has the M-Sport package but nothing else and I seem to remember that with the AWD on the 550 that means nothing. I called my friend on the way home and said "Your car feels so much faster than mine" and he totally agreed. It also feels much more nimble. The 650GC xi weighs about 175 lbs more and is a lot longer, but to me, the 550xi feels about 500 lbs lighter.
On that note...I've always thought my 650 felt a little doggy and really needed to be run in sport mode. Upon mentioning this to my friend (before we traded cars) he said his car was not like that. Now I understand why. In regular comfort mode his car feels faster and lighter than my car in sport mode.
And regarding the steering - they seem very similar now that my car does not have run-flats. Prior to the switch to the Conti DWS, my steering felt tighter than his.
We have agreed to keep the trade going for a while longer. He did comment how much he is enjoying my car - so I need to dig into that a little more. I think he is just in love with the looks.