I'm running a 55Ah Battery in my e92 M3. That was the smallest capacity which could be programmed into the battery management system by BMW.
Works without any problems.
I just get the battery weak message from time to time.
But that's because I don't use the car as a daily driver - mostly just for tracking.
So the car sometimes sits 2-4 weeks around in the garage.
If you'll use it as a daily driver you will not get the message as the battery will be charged regularly.
But even in my case. Never had the situation where I couldn't start the car anymore.
The battery monitor which throws the message is very sensitive.
So even if you get the message you'll be able to start the car 5-6 times before it's really to weak.
In my opinion this is a nobrainer if you're looking to save some lbs.
But don't just change the battery. Get a cheap 55Ah from an Autoparts store and then let BMW code the new capacity into the energy management system.
55Ah fit's without any custom bracket directly into the existing holes.