if you are wanting to run really big batteries with minimal handling issues, your best bet may be to look at a caster racing fusion EX1.5R . They offer this vehicle in an RTR and Pro version.
it uses the saddle pack lipo setup, so you can run 2 3S batteries easily assuming they are ROAR dimensions (bigger may fit, not sure).
These vehicles are actually pretty tough, and are compatible with a lot of the Mugen MBX5 / MBX6 parts .
When they come available, you can also find them used for really cheap. One just recently sold fully loaded with tires,etc. on ebay for about $400 . and when I say fully loaded, I mean every race option and then some with parts, etc on it.
When running a single pack setup with an ultra heavy battery, you can get weird weight bias issues to that battery side. And the further out the pack is from the centerline of the vehicle, the worse that condition will be.