Power = current X voltage
Power is what it takes to move the car so if power remains constant then when voltage goes up, it takes less current to make the same power. Current is what heats everthing up.

My Mugen 5S 1700KV burns around 240mA/minute
My Mugen 6S 1400Kv burns around 190mA/minute

They are both faster than they need to be.

In addition as you go up in voltage & down in motor KV the car gets smoother. Nothing is smother than my 6S setup.