There's a few things at work here:
1) Lowered drag in ground effect, so higher airspeed theoretically possible.
2) Increased lift; it's doubtful that the increase in lift @ 400kias would be greater than full control authority.
and
3) Wingtip vortices are greatest in clean config at slow speed, so would therefore be minimized in this example.
Based on the above, it would seem that the aerodynamics of the situation would allow a greater speed within ground effect. But at 400 kias, there would have to be a very very fine control of the aircraft to avoid some sort of mishap.
Q: What is maximum airspeed for a 757 at sea level?