as long as you are using an 3S lipo (12V) and you don't mix up the polarity, everything is fine. (Personally I would not go higher than 1300 mAh for performance reasons)
There turns out to quite a variety of AR 2-compatible batteries on the market. Especially on eBay. "Buyer beware", and all that. I took the plunge and ordered a pair of 2500mA batteries with a 'balance charger' from a seller that appeared to have some good reputation. Should arrive in about 2-3 weeks via slow boat from China. I can report back if people want to know how these batteries perform.