Yes, that is an option too, but Noctua's fans usually come with Low Noise adapters (basically an in-wire resistor that lowers the voltage to 7V or 5V), so I was thinking maybe trying that one out and seeing if there's a noticable performance loss.
I can tell you that the airflow with a slow standard fan will be only a slightly fraction of that of the original fan. You should decide for yourself if you are willing to take the risk of overheating. I don"t actually know if there is a good overheating protection built in.
I use my APW3 until around 1.000 Watts with the slower fan, but it depends a lot also on room temperature and different factors.