According to the articles I've read, the engineers who examined the footage calculated that the relative speed of the foam when it hit the wing was about 500 MPH. They calculated this based on the known speed of the Shuttle and frame rate of the camera.

The Shuttle was going roughly Mach 2 when it happened, but you're right, it's the relative speed that counts. The piece of foam was pretty big and had a low density, so it would've slowed down very quickly in a supersonic airstream. It'd be somewhat like throwing a feather out of an airplane window.

Based on the footage, they estimated the piece weighed over 2 pounds. In their original calculations, the determined that the impact wasn't sufficient to cause enough damage to endanger the Shuttle.

Ever since I saw the footage, I've wondered if they considered the effects of extreme cold on the physical properties of the insulation. The insulation was next to cyrogenic propellant tanks, so it would've been at least a couple hundred degrees below zero. At those temperatures, the insulation would've probably been a lot harder than at room temperature. That additional hardness might've been enough to cause significant damage. There has been some speculation that the foam might've had some ice attached as well, making it both heavier and perhaps harder than anticipated.