When the foam hit the wing, did it simply disintegrate or did it gouge into the tiles? A big part of the answer to that question will depend on the physical properties of the foam. From what I've seen, at room temperature, the foam is quite brittle.

However, the foam wasn't at room temperature. It had been exposed to extreme cold for hours before launch. Extreme cold can change the physical properties of materials. I read a New York Times article a couple weeks ago that cited an expert on plastics and composite materials who claimed the foam would've been something like 6 times harder at cyrogenic temperatures than at room temperature. Unfortunately, the article is now in their archives, so if you want to see more than the two paragraphs provided from the link, it'll cost about $3.

There is also disagreement over whether the foam absorbed moisture while sitting on the launch pad. If so, that moisture would've added to the mass of the piece that hit the wing and it would've been frozen, making it a lot harder than foam itself.

The Shuttle tiles are, from all accounts I've read, quite brittle. They were designed to withstand extreme temperatures but they're easily damaged by impacts. In addition, studies have shown that certain locations are more vulnerable to damage than others.

According to this article, back in 1992, tank debris dug a 9 x 4 inch gouge in Columbia's tiles, and that piece was estimated to have hit with less than 1/5th the energy of the recent launch. The investigators are also looking into other possible projectiles besides the foam.

I'm not saying that the foam impact was definitely the cause of the accident. There is still a lot of investigation to do. All I'm suggesting is that the analysis is more complicated than your numbers suggest. Put another way, a one ounce gunpowder charge is more than enough to blow off someone's fingers. That might be more than enough to damage those tiles.