We're about to throw out two of the sheets, deeming them irredeemable. As I scrubbed and scrubbed and scrubbed, trying to save them, I realized that a sandblaster would do the trick perfectly. A little bit of WWHHHIIIRRRRZZZZZZZSHSHSHSSHSHSZZZZZZZZZZZZZZZ and you'd have perfectly cleaned cookie sheets.
Since we aren't likely to get a sandblaster, here's another idea. I haven't tried it yet, but I'm going to give it a go.
Warning: The dude has a massive sink which allows the cookie sheet to rest at the bottom. Pure fantasy! No one has a sink that big!