[VintageMA]

Please help me out on how you calculated the watt/hr rating you obtained.

For the two 5000 mAh cells you mention I get:
4S = 14.8V 14.8V x 5Ah = 74W/hr
5S = 18.5V 18.5V x 5Ah = 92.5W/hr

Personally I go for a different perspective on looking at the higher voltages and try to keep the battery pack weight and overall power about the same. You know for most lipos of the 25C and 30C that the energy density is about the same (ie: a single 5000 mAh cell would be equal to two 2500 mAh cells).

If you are running a 4S 5000mAh pack and want to go to 5S I would recommend a 5S 4000 mAh pack. Or for 6S a 3300 mAh pack.
4S - 14.8V x 5Ah = 74W/hr
5S - 18.5V x 4Ah = 74W/hr
6S - 22.2V x 3.3Ah = 73.26W/hr

All of these setups will give you about the same runtime, but the 6S may give you a little more than the 4S. The reason is that if it takes say 1000Watts to move you truck then it will take ~33% less Amps at 6S than at 4S to produce the same 1000Watts.

Your batteries will run cooler as well as your ESC and motor with less current flow. It is the voltage drop across the purely resistive load of each component that produces the heat. And with lower current flow, the load will be easier on the cells and you will see less voltage drop at the cells themselves. Because of this the cells will be able to put out a higher voltage for a longer period of time where the higher current cells heat up faster and experience more voltage drop and give out quicker.

You may see that with a 4S 5Ah pack that you are only putting back in about 4500 mA, but running the 6S setup (say on the 3300mAh cells) you may find that you put back in 3200 mA because the pack ran more efficiently.