got rear-ended ; hidden damage?

so yesterday on my way to the NASCAR race in Loudon, NH there was a big back-up of stop and go traffic, and the driver behind me failed to stop when the traffic stopped . . . with a big thump and a jolt to my car. She was going real slow so it was not a major catastrphic impact, but a good solid thump.

The rear end of my car looks mostly OK. There is a scratch/dent on the plastic lower facia. The license plate is bent in on the bottom, which shows that the whole thing bent in quite a bit on impact. The body-color bumper cover seems to be OK. Looking under the car I cannot see any major damage; but I need to take a better look.

I'm thinking this is a good opportunity to get that Roush rear splitter (or maybe GT500) and I don't want to make a big deal out of the whole thing, but I want to make sure I am not overlooking anything.

Should I expect that there is damage underneath the bumper cover? Is there some kind of energy absobing material under there that would be crushed, and needs to be replaced? Is there any way to tell without removing the bumper cover?

update: I talked with my local dealer's service manager; and he said
that if it was 5 mph or below there should be no damage at all (except
maybe superficial scratches) and if I give it a real good visual
inspection, including under the car, and it looks OK, it probably is OK.

so I think I'll try to work someting out with the driver who hit me so I
can buy the Roush or GT500 splitter, instead of going through the
insurance mill to get a new stock GT facia. Wish me luck!