It may be a personally perceived difference to be honest with you, and I don't think that there is much of a difference in a real world scenario, but where I felt the 550 was really let down is that what the 850 got mostly standard (the Merino leather as an example) were all check box extras on the 550, including the integral steering. I suspect that once you ticked all those boxes you were getting pretty close to M5 money and original owners weren't prepared to go to that extreme. Not taking anything away from what the 550 is though, it's still a great car, just wasn't for me I guess!