There was a youtube video I came across a few weeks ago which took Usain Bolt's world record and did a bio-mechanical analysis to see how far off he was from his absolute theoretical best. It was... surprisingly close. I think they had him at like an 8.9s 100m if he did everything perfectly.
For those who don't know the current records well, from Wikipedia[0]:
The men's world record is 9.58 seconds, set by Jamaica's Usain Bolt in 2009, while the women's world record is 10.49 seconds, set by American Florence Griffith-Joyner in 1988[a].
[0] https://en.wikipedia.org/wiki/100_metres
[a] is a footnote on the wikipedia page, discussing the validity of the record due to wind speed measurement concerns; read the page and footnote if you care about that level of detail
It will be interesting to see what records the Enhanced Games (where athletes are allowed to take [more/different] performance enhancing drugs openly) throw up when they are held.
You'd need to persuade someone already at a pretty elite level to take part though. Maybe someone who just did their last Olympics?
They may do better than unenhanced humans (a fair number of eastern bloc records from the pre-testing era still stand today, especially in women's throwing events), but they're not going to exceed any hard biomechanical limits. No drug can grow more type II fibers or change the firing rate of the nerves.
The one extreme option out there is surgery to change the attachment point of the tendon to the bone, generating more torque from the same contractile force. Some possibly acropyphal rumors claim lifters in nations with very unscrupulous doctors may have done that when the opportunity presented itself because of an incidental muscle tear.
I agree in that they won't make bones tougher or tendons more elastic. They maybe able to add more TypeII fibres though if they grow additional muscle?
The other big thing on the horizon is gene doping, such as with Myostatin in Bully Whippets.
