Is it a GWS A-10?
If you want to stay with a 6 cell 7.2v pack then get a pack with a higher MAH rating.
You can try a 7 cell (8.4volts) or 8 cell (9.6volts) pack and you should see increased performance. Just make sure your ESC can handle the extra amps.
I have bought packs from
www.allerc.com I am very happy with the packs and the service was great.
Check their packs - All the pack dimensions are listed and you can get one close to the size of the ones you have now.
I use the 9.6 v 650mah Nimh in my GWS Zero. Awesome. They are 2/3 AA.
Hope this helps a little.