I agree it would be nice to drive the FETs at 12V, but with some of the newer FET's it doesn't seem necessary. I'll be using the IRLB 8743 which seems to do pretty well at 5V. It would be better at 12V but I'm not sure if I have space for the extra circuitry without a lot of changes to routing. I'll have to think about it for a future version.
The other consideration is that if you do that, then your board is limited to 12V on the heaters by the FET's as 24V would exceed the max gate voltage for most of them.
The other consideration is that if you do that, then your board is limited to 12V on the heaters by the FET's as 24V would exceed the max gate voltage for most of them.