Higher voltage and efficiency

Nobody is saying that as your wattage needs increase, you shouldn't go up in voltage. But none of my systems average even 20 amps. Yes there are points that go much higher, but on average between 12-17 . So I'm sure someone can throw the math to it, but assuming a 15amp average draw in my E-revo on 4s, for 45mph, What would really be the gain going to 8s? Is it fair to go by the average amps?