I have the BMP 9 pounder, and driving in traffic is no problem other than not being able to go anywhere. To me, the chatter is very minimal and I hardly hear it with the stereo on, and I don't have the volumn up very much. As for stalling, that hasn't happened to me yet! This is by far the best mod I did for this car. If it's a matter of a chip or FW, I would go with the FW and get the chip later. Driveability is not affected IMO, and I've had it in my car since Feb of this year. However, the FW is made for the M42 and the OEM clutch, so I don't know what problems other versions will manifest.
Since I had the FW, I was able to take my class at autox rather easily, and I also had a 1 sec quicker time over an M3 during track day. The car spools up very quickly, and you can actually feel the pull. My only regret is I waited too long to do this, and our track has been closed since April!
