I don't think that will be enough metal to passively cool it if the fan were to fail. These LEDs get hotter than I figured they would. I have no idea how people are cooling the 100 watt versions that some places sell.
I see no reason you cant under power the fan a bit. You just have to make sure it gets enough power to start up. Thats why a lot of pc's will start with there fans on high and then they drop back down to a slower speed. I would think if you ran it at 9v-10v rather than 12v it would start just fine.
I'm thinking I may mount the led drivers with rubber bands to the heat sink. I have a splash guard so if it falls of it should not hurt any thing.