I converted my Mugen Starter Box to LiPo as well, using two Hobby King Flightmax 4000mAh 2s 25c hard case packs. I have to admit, I should have done it earlier. It's truly amazing how much more power my box has. I no longer fear breaking in new engines (although I do pre-heat them of course).
Nice side-effect: these packs can be run in my Associated B4 buggy, too!