I bought my e90 with 50k on it and I didn't know I had the BMS until 10k later when I was poking around. I then tried "tuning" on/off and the different throttle remaps, and my experience says no perceptible difference in power. It runs richer and I believe it advances the timing slightly. In my head this tells me the gas mileage should average out, which is what I noticed with the way I drive. I ended up leaving it in (lazy) but running at stock because I didn't like the throttle remap either. Too twitchy for my tastes.
I'd say hold off for the alleged SC for the n52