Well, xfeed, I had vaguely in my memory that the 757 approached the Pentagon at high speed, so I readily accepted the 400 mph I found somewhere. About the same speed you mention.
The difficulty of controlling the airplane near max speed mainly depends on one’s practice, I would think. It is quite standard to go fast during part of the descent. Usually on autopilot.
Doing this at low level and manually is not on many people’s repertoire.
BTW, I 'll soon be offline for the rest of the week, between two 100 mph cruises at 100 feet below sea level.