Good article on the subject from 2011:
https://www.drf.com/news/jerardi-top...res-thing-past
"What I found out was the Beyer Figures by the top stakes horses were relatively similar from 1992 to 2005. And then it all started to slow down. With a few exceptions that don't last long (Uncle Mo), the best horses just keep getting slower on the Beyer scale."