Top end won't be that diff if you stay with 2S lipo, you'll just keep that fresh of the charger speed for a lot longer using lipo. I have an older EMaxx and got it ready to run lipo, just need the batts. Will be using smart stop for 2S on the batt that runs the BEC, rx and servos since its being drained a little faster, that'll protect the other one automatically.

Run time is most of the reason I'm doing it too - I get ~15min on 7 cell NiMH pacs and have been told to expect 20-25% more time for same capacity. I'm buying bigger capacity at the same time (5000mah vs 4000mah) and expect to see 25-30min or better.