I've just done some very unscientific testing with a bag of rice of 295gr and the car seems just about perfectly balanced front to back.
The 5500mah lipo pack I want is 295gr (
http://www.maxamps.com/Lipo-5500-74-Saddle.htm) so I don't think there will be too big an issue.
I'll report back when the batteries arrive and are fitted.
