There are about a million threads on rctech about lipos and the various applications people are using them in. Have a look at the search facility and you'll be amazed at what info you'll find. Many people will argue for a particular brand or make of pack and others another. You need to read up on the theory because the higher the capacity the pack and higher the discharge rate (C rating) of the pack then basically it'll hold voltage for longer. There is little point trying to find a 2500mah pack with a decent enough discharge rating for your application as I doubt these are readily available or even exist.