Actually seems like even against a 40 G capable missile the Cobra will be pretty competitive. Assume the worst case -- the missile is 1 meter behind the aircraft and is about to explode. The pilot pulls a Cobra.
Remember that the acceleration experienced by the plane and the missile are very different -- for the aircraft it is flying at high speeds and then exposing the entire bottom of the aircraft to the airflow to stop it (plain deceleration) . For the missile it has to do a full circle turn.
If you assume that the missile is flying at barely supersonic (assume it can slow down from mach 2+ -- it is easier to turn at subsonic speeds ) and that it can take 40 G (400 m/s^2) of centripetal acceleration, it will take about 9 seconds to do a full turn. If the human being can take 10 G for a few seconds, then the aircraft can pretty well do a full stop within that time. The aircraft would have done a pretty full stop, while the missile would be about 2 kms off by this point. Now the missile can cover this 2km within a few seconds , but the aircraft gets time to react .
Basically the whole move gains about 10 seconds (Assuming that the missile was about to explode when the pilot used the cobra). But I agree, it will be rarely used against missiles. More likely use is against other aircrafts in tail fights. Nothing else stops an aircraft faster.