a Duracell Alkaline AA cell is about 2800mah. so roughly the run time you get from a 2800mah NiMH sub-c pack, or take the run time you are getting for a 1600mah nimh pack and double it.
remmber that the cell connections and properties of the wire used to inter-connect something like that will become significant if your currents are more than an ampere or two.
Higher voltages parallel connected using diodes and a centrally located BEC device to knock the voltage down (and current up) might work much better.