Only if you don't size the wires and connectors to the current. If you use wires that are 40% larger in area (basically, 5/6 larger in diameter) and connectors to match, the I^2*R losses will match. ESC losses may or may not be higher at lower voltage/higher current. It depends on the ESC.
I don't think that I^2*R losses are going to be an issue unless the wires are seriously undersized.
What you say is absolutely true, one could recoup power lost to the supply by increasing wire size. The opposite is also true, one could reduce the wire size by increasing the voltage. A 40% change in wire size change can go either way. Going from a lower voltage battery to higher voltage battery would save 40% in wire weight. Visa versa. Going to lower voltage battery would, as you suggest, increase the weight by 40%. The planes and power systems we fly are generally oversized and can handle some small inefficiencies.
12000rpm / 18.5=650KV Motor
12000rpm / 22.2=540KV Motor
The mAh and C rating of the pack depends on the demand from the motors.
That's also basically correct but leaves out one aspect, the power required to run a propeller. For instance, it takes 250 watts to run a 10x5 propeller at 10,000 RPM and if we chose a motor than can only run 180 watts continuously, the motor would ultimately fail. So, we need to look also at the power rating for the motor. Generally we need about 20% - 30% more continuous rating, in this example we would want a motor with 300 W to 350 W capability.
There are a number of combinations of battery and motor to run that propeller. But that isn't all the battery has to supply. The battery has to supply the watts for the propeller plus the watts lost to heat. That heat is the current squared term. When we increase the supply voltage, that current is reduced and consequently the power lost to heat is reduced which in turn reduces the power the battery has to supply. The power lost to heat is somewhere around 10% - 20% of the total power, often a lot more. This means that by increasing the voltage you can save weight in both the batteries and the wiring.
This is an exercise we go through all the time in full scale aircraft installations. It is why you see 28 volt systems instead of 12 volt systems. On one jet I did, we saved nearly 300 pounds in the powerplant starting and control system. We spent a lot of money optimizing the starting system and ended up with 2 28v batteries that could be switched from parallel connection for normal operation to series for starting. There's more to it than the simply wiring in that case though involving the toque and speed of the motor.
Basically it takes a fixed amount of work to start an engine in the same way as it takes a fixed amount of work to fly a model through the pattern. The battery has to supply that work plus what ever is lost to heat. The batteries ability to supply that work and its weight are a direct function of its chemistry. The more work required means a larger amount of stuff the battery has to have. We could package all of that stuff into one package, ie one cell. Or we could package it into many cells. Since the voltage is also a function of the chemistry, we can arrange multiple cells and increase the voltage which reduces the losses which means we don't need as much stuff to heat the environment. This means our battery can be made lighter. This is a double win. We win because the wiring is lighter and we win because the battery can be lighter.
Practically speaking though, our batteries are limited by cell size. This doesn't truly allow us to fully optimize the power system. An example is for a typical small airplane such as a ringmaster or flight streak. We could use either a 2200 MAh 3s Lipo or a 1800 Mah 4s Lipo with the same exact motor and the performance would be practically indistinguishable and the weight difference minimal. What we would see operationally, is that the total Whr we put back in would be lower in the 1800 MAh battery or as people like to say, how many MAh put back in.
The other aspect we would notice is in the longevity of the batteries. Reducing the current and therefore the heat production includes what each cell sees. Less heating within the cells means the batteries will last longer. For many people this is irrelevant because the batteries often have a different fate that ends their useful service.