Actually, Sachs, I believe it would make it in well under 12 seconds. I can't post a link here, don't know if it's ever been on the web, but in 1985, just for the heck of it, Dirtbike magazine did an article pitting some hot (at the time) Honda street 500 against the '85 Honda CR500. They put street tires on the CR, raised the gearing, of course, and I believe put a wheelie-bar on it as well, but the motor was stock, as was the frame and suspension, carb, and everything else. It easily smoked the 500 streetbike and came in the high 11s in the quarter mile. The '85 Honda CR500 was one of the more powerful versions and would easily beat say a 2000 or 2001 CR500 which in comparison, were a bit de-tuned. Going by that, given the same mods, and a decent rider, the KX ought to do slightly better even than that bike. Anyhow, high 11s in the quarter mile should be easily achievable on a good running KX500 with a stock motor.
As to that YZ vid, that was most impressive. I wonder what motor modifications it had. Most of the 500 2strokes were very mildly tuned, compared to say, a 250, and I've always wondered what one could do when somebody really hopped it up. That YZ 490 video speaks volumes!
