Can I skip the MEGA 2560's regulator to supply 5v?

I have overtaxed the power supply for the MEGA with various 5v components. One happy accident helped me figure that out. An ESC I was using in testing has a BEC that was feeding me +6 when I thought it was requiring +5, which is the case with the "real" ESC on the prototype. Components I had been having trouble with began working perfectly when I had that ESC hooked up. I am getting a 5v regulator with more than enough amps to go around. Anyway, is there a reason to use the Arduino's regulator at all? Looking at efficiency graphs (at Pololu, where I ordered the new regulator), the more power I draw through the regulator, the more efficient it becomes. I know feeding the MEGA VCC works, from my accidental discovery with the BEC on the test ESC. When it was connected, the Arduino ran fine even with nothing connected to USB or the barrel jack. But I don't know if that was a second stroke of luck or not. Is there a reason for it to use its own regulator instead?

Bypass away! I bring in 5V to my arduino's and the rest of my circuit all the time. The Power pins don't care where the 5V comes from.
These 5V, 4A supplies work very nice:
http://www.mpja.com/prodinfo.asp?number=18520+PS
Install a panel mount powerjack and away you go!
http://www.mpja.com/prodinfo.asp?number=18549+PL

Actually, I will be going DC to DC with this:

I figured it would be okay but I got a little concerned after reading a comment on the Fio product page at SparkFun. I also have a Fio; that is not what this question is about, but I was concerned there might be a similar issue. Anyway, the poster lamented the low amperage available on that board and said that due to the way it was regulated, he could not simply bypass it. I am not sure why or if he is even correct, but it made me worry that the MEGA might have a similar issue.

That's fine - I was using one of those to go from 12V to 5v, until I figured I needed a lot more 5v current than 12V, so I got a beefier 5V supply and a 12V upconverter for the smaller amount of 12v current I needed. Pololu Adjustable Boost Regulator 4-25V

Well there is always the issue of when you plug in USB to a PC you will then have two sources of +5vdc wired together. Not a recommended practice, but often done without reported problems. I would at a minimum wire a series diode from your external +5 voltage source and the Aruino boards +5v bus for some isolation.

Lefty

I pull the prominis to program them. Having Rx/Tx connected to other stuff messes up the FTDI comm's, so converging supplies is not a problem for my designs.

two sources of +5vdc wired together. Not a recommended practice, but often done without reported problems.

Are there ever problems? I was accidentally wiring 6v to 5v, but that’s just like having batteries in parallel with one drawn down a little more than the other. I know there is the logistical issue (in some cases I guess it could be a problem) of easily controlling the flow; either source can turn it on but neither can reliably turn it off.

Are there ever problems? I was accidentally wiring 6v to 5v, but that's just like having batteries in parallel with one drawn down a little more than the other. I know there is the logistical issue (in some cases I guess it could be a problem) of easily controlling the flow; either source can turn it on but neither can reliably turn it off.

Well it's a little more complex then that, but your right that turning off power could be problematic. Say you are plugged into the usb and turn off the PC, will the external +5vdc then try and keep the PC powered on via the USB power pins? Hint the on-board thermofuse would prevent that but is that a good condition to leave the system at? What happens if you power off the external +5vdc, will the USB power try and maintain current and keep the Arduino running and trying to power loads? Again the thermofuse will limit current to 500ma but again is this a good condition to leave a system?

Again it's not automatically a dangerous condition, but I suspect you would never see a commercial design wiring two independent active voltage sources directly together without some kind of isolation.

Lefty