I've had a real positive windshear at 39000' over Paris - we went from a 40 kt tailwind to a 20 kt in a couple of seconds whilst flying at M0.78. The most effective, safest initial action is to pull speed and select it back (this changes the thrust mode and ATHR is far more responsive. Then we slowly used some speed brakes. We didn't overspeed!
Not an Airbus pilot (my airplane doesn't even have A/T), but why wouldn't you just reduce power manually? Not criticising your action, as I'm not in a position to do so, but genuinely curious.