I know that going from a 7 cell nimh to a 2s lipo will get more speed do to the higher discharge rate and they way they discharge, but what i am curious about is if everything else in the truck stays the same, will a lipo with the same mah rating give more run time than a nimh
In my experience, yes; but not much.
If you think about it, a portion of NiMH's energy is spent in heat... the packs get warm during use. LiPo's remain relatively cool.
Whatever it is I just typed... could be wrong.
As long as the motor/ESC system does not over-tax the Nimh battery pack, in other words the Nimh pack is a very good pack capable of handling the loads asked of it - then the runtimes between a similar sized 2s LiPo and the Nimh will be nearly identical. Nowadays the problem lies in finding really good Nimh packs, as I don't know of any company offering high end matched packs for sale.
The heat difference noted by Jimmy is partially due to individual cell IR's. When they were available (certainly not now), good quality Nimh cells had IR's that were lower than any RC LiPo cell on the market and only just recently do the very best LiPo cells have IR's this low.