OK first of all you need to keep in mind that it is the voltage of the battery that gives you the power and the capacity (run time) is the current (mah). The good thing about LiPo is that the discharge curve is nearly flat or in other words, it puts out about the same voltage at the end of the run as it did at the begining. Whereas NiMH has very good initial punch but tapers off as the run progresses.

To answer your question regarding the Stampede, it will run about the same perhaps a tiny little bit faster but not by a lot with a 2cell LiPo over a 6 cell NiMH (7.4volts vs. 7.2volts nominally). What you need to look at is the continuous discharge rating that is more important than just voltage or capacity. A discharge of 25C is very good and suitable for stock motor racing. A 20C, 15C or 10C means that it discharges softer and lacks the punch of a 25C or higher. So this is the real measurement to look for if you're after performance. A 3200mah 25C pack will outperform a 5000mah 15C but will run a shorter amount of time under the same load.