I distinctly recall a gamedev.net discussion on glReadPixels suddenly getting really slow on catalyst 9.1. I also *think* that FireGL cards where not affected, indicating a "product segmentation" reason for the slowdown, rather than a technical one.
Unfortunately, I haven't been able to find the link, so I could be wrong (if I am please shoot me!)
Interesting, I just noticed one of the few fglrx bug reports that actually did get some feed back. And it looks like you are right. They intentionally crippled fglrx glReadPixels performance for consumer cards.