Thanks Megan for the very comprehensive answer.
The reason I was asking was this video below. (Which you've probably seen) The Boeing pilot says early on that they would be making low level 10minute runs at 280' 350 and 400 knots calibrated. All the civilian Boeings that I flew were limited to around 350knots indicated. I can't remember if it was exactly 350, it might have been. (737, 757, 767)
400knots indicated seemed very high, but you have confirmed that it was close to that, at 391 indicated. What was the noise level like at that speed? I wouldn't have felt very comfortable at 500feet at redline, I must confess!