My point was that you let the aircraft develop a sink rate of 1600fpm. Unless the speed suddenly dies in the @rse (you "lost" 25 knots over around 45 seconds [600ft at 750fpm, maybe less time because of your increased sink rate]; I wouldn't call that a really bad windshear), the A/THR isn't going to jam on a heap of power in a hurry. Now if you'd pulled back on the stick smartly to stay on the slope/maintain the approach sink rate (dunno what Vls is but say 150=750fpm) the speed would have dropped off at a greater rate and the ATS would have kicked in earlier.
In my machine, a significant undershoot shear corrected by a good pull on the stick would have the ATS (throttles here) jumping out of it's skin to correct the speed reduction.
I'm wondering if your A/THR would have reacted quicker if you'd reacted sooner to counteract the undershoot shear?