<I wonder exactly how much energy would have been released by the foam hitting the tiles?

That depends on a lot of factors that the engineers are struggling to calculate. These include:

1. The mass of the foam piece. Based ib the news stories I've read, current estimates are on the order of 2 pounds, give or take a bit since they don't know the exact size of the piece.

2. The relative velocity between the foam and the wing. I've read estimates on the order of 500 MPH impact velocity.

3. The angle of the impact. A glancing blow mignt not do as much damage as a direct hit, but a glancing blow could possibly damage several times at once.

4. Here's one I don't know if they're considering or not: the hardness of the foam. According to a diagram I saw this morning, the foam is believed to have come off of the ET near the top of the liquid hydrogen tank. Liquid hydrogen is over 400 degrees below zero Fahrenheit. While there could've been some aerodynamic heating on one side of the foam, the side near the tank was likely very cold. I believe that the foam was probably very stiff, perhaps even hard as a rock, due to the cold. If so, I believe the foam would've done a lot more damage than their initial calculations indicated. This could be significant. Remember the Challenger investigation: the cold made the rubber O-ring stiff, reducing its ability to properly seal the gap.

Remember the punch line from the urban legend about the chicken cannon: "Thaw the chicken."

There's also this based on a 1994 study as reported by Florida Today: some tiles are apparently more vulnerable than others.