Ok, I've been doing some testing to find out just how much load a 3A power supply can handle before creating brown-outs on the logic pwr bus. What I'm finding is that certain types of servos really pull some current when they move quickly regardless of their torque ratings. For instance, I'm testing with some Hitec HS605MG's that are just absolute power hogs even though they're not outrageously powerful torque-wise. The digitals I'm testing with have twice the torque or more, but have way lower power consumption spikes when moving quickly. So the moral of the story here is that we just can't equate power requirements to torque.
So to get to the point, it's going to be trial and error in terms of figuring out whether you'll need to go with dual power supplies or can live with a single one. If you want a recommendation, I'd say 4 servos is the limit for using the single power supply configuration. Although I can get a brown-out to occur with just 3 HS605MG's by putting the DMX tester in "pulse" mode (toggles all channels full on/full off).
I'm currently lobbying for a minor change to the PCB design for the single/dual power input option. I'm hoping we can allow for switching between the two with maybe a jumper rather than soldering a diode in one of two places. That will make it much more flexible and allows you to change back and forth between single and dual in an instant. Will keep you posted on that. The other minor change is to feed the Ex/Rx with 6v from the upstream side of the logic pwr bus regulator. That will help raise the tolerance a bit for power dips.
So in summary, using dual power supplies is working flawlessly. The questions only arise when using a single power source for both logic and servos.
Dave