What you should really test is whether Tornadus/Thundurus pre-KO and post-KO are identical to each other. If they're different, then the game rerolls its stats, which would also include its shiny chance; if they're the same, then it doesn't.
I'd test it myself, but I've already caught Thundurus in my copy of White, and I'm not completely resetting just for an answer.