Well...You can go as high as a 3000MAH NiMH battery pack, which will give 2+ hours of runtime, but it's not compatable with Streamlight's chargers.
There's some math that goes into charge rate vs. capacity, and if the battery will fully charge. Streamlight's chargers can accomodate a 2400MAH battery pack (vs. the stock 1800MAH pack) which yields about 80 minutes of runtime.
Currently, no company offers a high-cpacity pack for Streamlight's or Surefires. You have to make your own, and there's a bunch of info. on CPF about it...
P.S. I'm moving this to Batteries...
--dan