I don't have personal experience, but the the GSCB symposium a couple years ago I talked to Dean Pappas about it. Running the smaller motor at a higher power level was one thing he was thinking about. Save a couple ounces on motor weight and run higher voltage. As Mike says, the problem would be keeping the motor cool, but the couple articles Dean had in FM last year showed how to do that with ducting. You can't just hang the motor out in the breeze. In needs a cowel designed to force cooling air through it.
I have looked at many different power system configurations from low voltage/high amperage to high voltage/low amperage. There is no clear winner when it comes to weight.
If you draw more current through a small light weight motor you need a heavier battery (due to added capacity) to get the flight time. It will also turn more energy into wasted heat.
If you run higher voltage through a small light weight motor you again need a heavier battery (due to the added voltage) to get the power back. However, it will more than likely give you more power to the prop for the same weight as low voltage/high amperage setup (less power turned into heat).
The best combination is the most efficient combination. Using the most efficient motor for your application is going to give you the best results. Some brushless outrunners and cheap Chinese motors have surprisingly bad efficiency (60-75%), nearing the efficiency of high-end brushed motors. I believe most of the AXI motors run in the 80-88% efficiency range. There are motors out there capable of 92-94%. Couple that efficiency with a high voltage/low amperage setup and you will turn less of the available power into wasted heat.
No matter how efficient the motor is there is a point of diminishing returns. You won't be able to get rid of the heat fast enough and will have to move up to a larger motor.
The motors and batteries available today are already capable of competing with the best IC engines. As mentioned before you need to get away from the heavy ARF's and scratch build. The fact that you don't need an engine crutch, plywood doublers, and a tank floor/mount makes the power system weight almost equal to an IC setup. Properly applied, the power system technology is here.