110V vs. 220V
You probably don't want to hear this, but:
First, the internal windings of "small" electric motors run on 110V even when 220V is supplied to the motor's wiring plate.
Also, it's a common misconception that a motor running on 220V use less current and will generate more power than the same motor running on 110V. It won't.
While it's true that the motor will draw 15A at 110V and 7.5A at 220V, that 7.5A is per leg, and there are two legs (two hot wires), therefore it still uses the same 15A. At 110V there is only one leg (one hot wire), hence the 15A.
As to power, Power = Volts x Amps. So at 110V and 15A, the power is 1,650 Watts, or roughly 2 HP. Given that the houose voltage is probably not a constant 110 volts, and in fact can did as low as 90-odd volts, the voltage supplied to the sub-panel will also be less than 110V. Couple that with any small IR drop in the wire, outlet, cord, etc., you can see how the voltage could be significantly less than 110V, and so the "power in" would be lower also (and the output power cannot be more than the input power).
For 220V Power still = Volts x Amps. Double the Voltage and halve the Amperage and you still have the same P = Volts x Amps, or 1,650 Watts, and the same roughly 2 horesepower. You can't increase a motor's power by rewiring it to run on 220V.
To sum it all up:
1. The motor's power will be the same whether wired for 110V or 220V.
2. Using 220V allows the use of a smaller wire (not a significant cost saving).
3. 220V can be wired from a single douoble pole breaker. Note that I said double pole, not two pole. There is a difference.
4. There may be a tiny reduction in total circuit amperage, which will give a tiny reduction in Watts as recorded by the electric meter on your house, but it not even be measurable or noticeable on your bill.