Please think carefully about changing out to smaller capacity battery on E9x M3
S65 engine DISCONNECTS alternator while full throttle...
Which means the engine only runs on battery power on full throttle.
I am sure alternator will kick in when it is under the required power to run the motor...but then what's the point of having lighter batt?
I honestly do NOT know how long will this Braille battery will stand, but I would be very careful switching into smaller capacity than stock just because of above reason.