It all really depends on track and driving style. As of recent I've been using just about all of my 8200 mAh in 7min (7849mAh 2 weekends ago).
If you can find them in stock I think that the best bang for your buck (non ROAR) is the SPC Racing 2s2p 50c 8200mAh pack. Especially since some mains around here are 10min.
http://spcracingbatteries.com/index....&product_id=66