I have a monster GT I converted to brushless.

When i converted the truck to brushless I was running 6 cell NiMh batteries in it. Run time wasnt bad, and power was good, but the batteries would come out of the truck smoking hot. Basically It took a LOT of life out of the 3 sets of batteries I was running before I went to LiPo's. Now with the LiPo's the batteries run cool as can be, and the run times and power are better.

How hot are your batteries when you finish a run? If they are pretty hot, then you know you are working them hard. If you drive fairly easy they may come out just warm, which is fine. But brushless is definately more strain on them and lipo is the best way to go. However, you already have the 7 cell's, worst you can do is run them till they are dead, then buy lipo's....