C'mon guys!
Mass x Velocity
A piece of foam has little apparent mass but significant surface area. When exposed to a supersonic windblast it accelerates VERY quickly. For something to puncture the composite leading edge and do that much damage, it had to be moving quickly.
The Shuttle was moving well in excess of 500mph at the point the foam departed into the slipstream. Also, consider the venturi effect of the main tank and the Shuttle underside. There may be some dynamic there which produced more or less wind velocity than the actual true airspeed of the vehicle.
PT