anzer, one thing I should point out - it's one thing to say there is a little lag off the line. Even with the minimal lag I experienced (whatever the reason), I would certainly not describe the 50i as "laggy" which implies that lag is an issue with the car which it certainly isn't. Whether or not there is lag, it is nothing to be concerned about.
BTW, the only other mention to lag I've seen was from a X6 50i owner in response to my test drive post:
http://www.xoutpost.com/739989-post4.html