You could also try a 6 cell 2/3 A NiMH pack. That way you probaby wouldn't need a dedicated LiPo charger (if you're not running them already). They have plenty of go with those type of cells.

It also depends on what motor you're running. The kit motor with no labelling doesn't have much go. The Losi Speed Motor is a bit better, but wears out quickly. The Trinity Mini Monster is a bit quicker again and has replaceable brushes. If you want to go insame and destroy your gearbox while your at it, a brushless system will have plenty of get up and go.