You are on the right track, and your general conclusions are correct, but you are messing up the units/terminology a bit. Power would just be the wattage, but you are calculating the Energy of the batt, either Watt-hours or Joules (=> ~213 kJ.) Ie Energy is Power over time.
You will be expending the same amt of E & P regardless, P is what moves the truck, its just you will be building P with V instead of A. You know this tho, as you are trying to find a batt with the same Power (output potential) and Energy (work capacity) but higher V.
First, you would have the advantage of the motor running more efficiently, requiring less current
One note tho, I don't think its important to make a big deal of the efficiencies. Both motors (of the same line) can be highly eff, but the lo kv motor will better by a ~couple %. This is not going to make dramatic differences is heat generation. What's going to make the greatest diff is the amp reduction, as heat gen is directly related to current.
Look at the data @ Lehner comparing a 2000XL @ 22V vs a 4200XL @ 11V. This would be comparable to converting a 3S system to a 6S system. The power output is about the same, the rpms are ~same, and even the eff are only down a bit across the board. The huge diff is the amp draw. Its half, thus heat gen would be ~half.
Nor would I assume a motor running at hi rpm is going to be in-efficient or automatically undesireable. Again the charts. Also see the one for the 1950 6T/ high amp. Its over 94% peak eff @ 68K rpm. Its actually more eff than a 1950/11 turn running at same voltage and ~1/2 rpm. However, for a given output in our range (~1500W) the 11T is more eff, but the current draw is the same. Tq is ~half go gearing should about follow. The end result looks like they should both be about the same speed wise, but the 11t run a bit cooler.
=> New high current/ high voltage controllers should make motor choices interesting. :)