I currently have a 535xi with a tune and I am starting to regret my decision to not get the 550 in the first place. Again the 535 isn't a slow car by any means. Its quick around town but where i find it lacking is on the highway once you've already reached a steady cruising speed. When i punch it in my 535 the car moves but doesn't have that "I am about to blast off rocket" feel that i use to get in my tuned 335 pushing over 400hp/tq. Its much more tame in the 535 where as my 335 made you feel like you were about to take off sucking you into your seat.
Never test drove the 550 but i'm sure it will feel pretty similar to my tuned 335 if not faster.