Are you looking for the same size battery or extended (bigger mah)?
If extended just get zerolemon, value for money. Will last you for days of heavy usage.
Someone used it and posted in this forum a few (few few few) pages back, got link in ebay too I think.
Have a look at that, price should be around RM160+ if i remembered correctly.
FYI it's the zerolemon 7000mah
That would be the best way. With custom ROM (some) have different kernels that will greatly effect speed and memory management. Now you have to choose either to flash custom stock a like ROM or AOSP or something else.. check the first thread.
Another thing, in regards to battery life - my experience with stock ROM has been great. When switch to custom ROM speed increased and I have more features but battery life not as good as before. I'm using UltimaROM now. Famous is wanamlite / Revolution HD and some other more you can check few pages back.
All the best!