If you're cooling the wick by whatever means, any wattage is theoretically possible. Mine starts to taste burned at around 10 watts, but my cooling is limited.
With a well-saturated wick that can stay well-saturated and plenty of air flow, the limit is much higher. Both the liquid and the flowing air cool the coil, and there's always liquid ready to be vaporized.
I'm waiting for the first RBA with a radiator system.
