But the piece of foam wasn't at Mach 2 relative to the Columbia wing. If you think about, it the foam had about 100ft +/- to accelerate from 0 mph relative to the wing to whatever speed it impacted at. That speed was certainly well below the speed of the entire shuttle/tank/solid rocked combination at that point. If I had to hazard a guess I'd say relative impact speed was below 200mph - but it could be determined accurately by time-lapse analysis of the film.
Someone did just that, and estimated it at about 125mph, plus or minus about 30%.
When you calculate the total energy, it sounds like alot, but so is a slap in the face.
I am sure NASA has the data on that by now. They are working round the clock.