[BrianG]

As you may know, using a switching type BEC is much more efficient than the one built into the ESC since those are linear types. Not sure if you wanted to figure out main amperage draw from just the BEC, but basically it's:

((servo_amps * bec_volts) / 0.85) / v_battery

figure 3A as worst case for servo_amps, bec_volts is 5v or 6v depending on the BEC, 0.85 is the average efficiency of a switching BEC but it depends on load and BEC quality, and v_battery is the voltage of your battery pack. And yes, the current from your battery will actually be less than the servo_amps.

A motors wattage figure is typically the amount of power the physical wires are capable of handling. However, as jollyjumper said, it depends on the applied voltage. You won't generate 2700w with 9.6v for two reasons:
1) power is the product of voltage and current and to get 2700w at 9.6v, you'd need over 281A. Most batteries cannot do even near this amount for very long at all.
2) the motor has a certain impedance of the coils, but this varies with load and motor speed. At stall, the impedance is just the DC resistance of the motor coils, which is VERY low. At higher rpms, back EMF will increase the impedance. Mush of the current draw will be as you are accelerating because the motor is trying to change speed. The quicker it tries to change speed under load, the more current will be required.

Because there are so many factors in current draw making it almost impossible to calculate, most simply select a high-enough voltage to spin the motor at ~35,000 rpm. Then, load the motor (through proper gearing) so the motor, ESC, and batteries don't get too hot. If you really want to know the current/power, it would be best to get an Eagletree device, which plugs in series with your battery to measure voltage and current (and calculates the power) at the battery. You can then upload the device data to a PC and see the voltage and current levels over time.