If I’m following correctly you are probably headed the wrong way for a lightweight setup. For a similar run time the lower volt higher kv setup won’t be lighter, as the cell capacity will need to be larger to provide the higher current needed to drive the higher kv motor and achieve similar power levels. It will likely be somewhat heavier even, as all else being equal it will be less efficient needing even more battery weight, and run hotter.

Better to go for a higher voltage low kv setup, within the limits of any possible racing class rules if you are headed that way.

As a rough example a 11.1V 5000mAh pack with the high kv motor could quite likely be replaced by a 22.2V 2500mAh pack with the low kv motor. Similar overall weight, with some advantage to the high voltage case even.

Are you planning on racing, general playing with lots of power, speed runs, etc? If racing do you know the rules if any at the tracks you plan on running at? That and any more details you care to share will help us provide better advice. Generally there isn't a way to calculate everything very well, due to lack of enough data on motors and such and too many possible variables in how the RC is used.