You're doing alright with that little battery for so many shots
If you're putting that many rounds through it...it's simply because it's a little battery.
They make 8.4v mini (that's the terminology for that pack size) pack that are 1400 and 1600mAh. There's also 7.4v lipos to look at.
Check airsoftparts.ca ...local to GTA and a fast shipper. Do a search for "charger" and take a look at either the B6 or X6. They'll do all sorts of batteries and I've found them to be reliable and consistent.
They've got all sorts of batteries too...stick with Sanyo and Intellect...avoid the green/black no-lable brands.
10C should be ok...small cells (2/3A vs. SubC) are affected more by temp.