If BMW says 5.3sec, I'm sure a real life tests will yield sub 5.0sec times! Motor Trend rated the Mustang GT between 4.9 and 5.1sec. Not that I would ever buy one! But imagine smoking a GT or a 335i in a SUV... and it's not even an ///M!
I took out an M3 with my 4.8i but that was because of an in-experienced driver! While he was busy trying to change gears, I had mine in sport mode with my foot down!
So now it's going to be a tough choice between upgrading to the 5.0 or the X5M. BMW did the same thing between the 550i and the M5 and probably the 335i and the M3 as well!