Stall torque (and current) and peak (no load) RPM tell the whole story of the performance characteristics of PM DC motors.
The torque output of, and the current drawn by, the motor vary linearly with speed - maximum at stall (zero revs), minimum at peak revs.
Therefore peak output power is at half peak revs.
Note that the stall current given is assuming a perfect voltage source. You may want to take into account the resistance of your batteries to get a more realistic output torque. In heavyweight robots, the resistance of the batteries is quite often at least as much as that of the motor (particularly with high-power motors like the S28-400), so will half the stall current and torque.
See http://howto.killerhurtz.co.uk/boschspec.gif for a typical motor graph.