> My supply sure looks linear, so I cranked it up to +12VDC.  
> Now, at 32C 
> ambient temperature, the supply seems cooler to the touch, 
> and the firewall 
> is working without a hitch.  Maybe I won't have to spring for 
> the better 
> one after all.


Any electrical device has a wattage that it requires to operate.
Wattage is Volts * Amps. 

As the recommended voltage goes down, the amperage goes up.  Amperage
causes excessive heat and can also fry things very quickly.

Running a device at the recommended voltage is the best way to go. It
will run cooler and more efficiently.

-Kevin