Nothing odd about running better w/new sensors. However the reads may have slowed or stopped (could have gotten clogged with carbon soot) because the cat may have died. You need to be able to see what the O2's are doing. The front ones should be switching and the rear ones should be steady.
Try the swap and see what happens. If it does the same thing you may have a toasted cat, if the code moves you may have a bad sensor.
Otherwise it might be time to take it in and have it analyzed by a BMW guy.