You also have to look at efficiency. The More voltage you have the less Amperage you will use for a given Power. P=V*I. Now if you go Up in voltage the motor's RPm will go up. That can be good but for racing it is usually kept about 30000 theoretical max RPM . Now to get longer run times with the same power as a 4S 2000kv setup with a 5S 1700kv or 6s 1400kv.
The 5 and 6S setups will use less Mah per min therefore longer runtimes on the same Mah batteries. But heavier batteries too.