It's a common mod of most XR's. There is a backfire screen under the filter and the idea is that it improves performance. The snorkle mod is also and XR thing but I've heard of it on some other bikes as well. Basically another airflow modification.
I know a guys 06 CRF450, dang near burnt him also. If you want a race bike, get one. If you want to build one, thats another story. Make sure the bike is in good mechanical shape, jet it, and ride it. Vintage Bob
Not so sure on the screen, though. I saw dyno testing on a YZ, screen removed vs. screen in place, and it didn't make any difference. Plus if you were to cut it out, you would have to be careful there wasn't any possibility of little "hairs" of screen left over that could be sucked in.