Please be kind...
My opinion is that the fastest anybody can run clean is somewhere in the mid-9.8's. Here's why:
Mid-9.8's are not that far from 10.00. Take a 10-flat guy, give him a tailwind and he's mid-9.8's. Guys have been around 10.00 for a very long time.
There are many mid-9.8 guys who never got caught--Thompson, Boldon, Surin, Burrell, Fredricks, Bailey, Fasuba, Obikwelu, Lewis, the list goes on and on. These same guys have many credible performances in the 9.9's in finals. For many of them, they approached the mid-9.8's only once or twice, when conditions were particularly good, including tailwinds.
Consider the guys who have gone below the 9.8's: Johnson, Montgomery, Gatlin (caught)--Greene (should have been caught)--Powell, Gay, and Bolt. Almost 50% confirmed dopers, possibly higher. Sorry if I've forgotten anybody.
Anybody know what percentage of mid-9.8 guys are confirmed dopers? I'm guessing it's well below 50%.
Put the best old-school runners on a modern track with modern gear and a good wind, and mid-9.8's would not be outrageous.
My 2 cents worth. By the way, I don't even know what to think about Bolt. Even if he is doping, a 9.58 likely translates to a 9.7-something clean. He really may be the fastest ever.