I can't comment on the -400, but I have been involved with airtests and instructing the dual engine failure on the A320. Sufficient hydraulic pressure, to operate flaps/slats, is available down to about 190kts. Although the systems normally operate at 3000psi, the pressure has to drop to approximately 1700psi before the flight controls are effected and the ECAM caution warns. The drop from 3000-1700psi takes about 30secs, with vigorous control movement, once windmilling has stopped.