Just what I would do:
Stick an ampmeter between your M2 and your mobo (probably going to have to cut something) and see how much your pulling with every thing on and running. If you have less than 3A draw on the +5v you can make the call to run it off the power supply or not, its your choice to live with.
If I were in your situation (I will be when I get back to my truck), I would look around here in the hardware fourms and see if you can find a simple +5v power supply. If you can build it, it may be your best option.
+5v is your normal +5v rail. +5vSB is your +5 stand by voltage, it is what powers your computer so that a PSON signal can be applied. Dont draw anything off of that, probably not a wise idea.
And your device will only pull as many Amps as it needs, the M2 can supply up to 8A continuous, but depending what you have on it, it may only be pushing 3A to power those devices.