[nuz69]

You don't drop the voltage, you keep it same, 4S for both 1515 and 1518.
Feel free to do the test, I have done it between 1515 and 1520, both on 4S. You just need a bigger pinion with the 1520 to have the same top speed, both motor operate at the same voltage and draw the same current, both motor provide same performance and consume the same power, the 1520 is just a little more efficient because it handle the same amp in a bigger space.
And the 1520 does not produce more power than the 1515, the 1520 "CAN HANDLE" more power if it "HAS TO", there is a difference.
It's not a Kv comparison like 1515/1Y 2200Kv and 1515/1.5Y 1500Kv, in this case, like you said, the 1.5Y windings need 1.5 more voltage to have the same performance. See the castle description for 1512 motor, max amp rating is 80A for 1512/1.5Y and 120A for 1512/1Y.