The RCX system provides a certain average voltage level (via PWM) to the motors. This means that speed can vary based on load, and may even vary slightly from motor to motor. Power will also vary based on load, as a motor with more load will draw more current if voltage is held constant.
If you're interested in specifics, you can measure the voltage and current on the motor lines with a voltmeter, or view it on an oscilloscope if you have one available. From there, you can calculate the power (P=IV) for your setup.
The RCX system does have a rotation sensor if you need to be able to make specific speed and/or distance adjustments. In later systems, additional data lines were added to the communication protocol to allow motors to have embedded rotation sensors and allow precise speed control, but this is not possible with the RCX system.