Having run the Deka ETX30L that Braille rebadges as its 21 lb battery in my 99M3, I would not run it in my E90M3 unless I lived in a warm climate and did not mind trickle charging it. I live in NH and drive my cars year round and do not want to be bothered with a trickle charger. The ETX30L lasted one year in my 99M3. It did make it through the first winter, but could not handle even the beginning of the second winter.
I pulled it and swapped in a Mini battery. No issues with that in a couple of years. I would consider trying one in my E90M3 although I would not expect it to last more than a couple of years. However, I am not sure I am desperate enough to save 20 lbs to buy a battery that will not last as long as a stock one. I have to think about it since I will be due for a battery -- my stock one is nearly 5 years old and that is a good lifespan for an E90M3.
Whatever I buy will be an aftermarket battery since I have no interest in overpaying the dealer and then arguing with them. This is a very common problem these days. I went through it with my wife's 08 535xi. The first dealer ignored my request, replaced a cable instead, and the battery died again days later. We had BMW roadside assistance tow it 40 miles to another dealer, which diagnosed it correctly and replaced the battery. BMW spent twice as much as it should have on that repair. I don't use the first dealer, Tulley in Manchester, NH, anymore since I have no patience for overpriced incompetence that wastes my time.