In the good old days of the Berlin corridors, we were restricted to cruise at 3000ft and latterly at FL85/95 depending on direction. The length of time in the corridors was 15-20 minutes. If we had cruised at 250kts the schedules would never have worked. We used to fly the 737-200's at 320kts all the time, don't recall having any birdstrikes, so I guess we were lucky!