I do think you can go smaller and still get by for a year or two. Something like the Mini battery will survive for a while. It weighs about 34 lbs. The stock battery looks to me like it is about 60 lbs.
I run the Mini battery in my turbo E36M3. It works. I tried a Deka ETX30L, which is the battery resold by Braille as their ultimate LTW battery. It lasted about one year of daily, year round driving including winter in NH. That was it. With all the junk on an E90M3 plus the very high compression V8, the 22 lb Braille would not last long. I would not bother.