I think a situation arises where, if you cannot cut something up and put it under a microscope to see what is happening, it is a tougher experiment to have data on.
I have also heard some gurus/scientists say, it is not necessarily the CNS, but the PNS that can be over taxed.
Questions also arise, that if you can train the muscle fibers & structural system along with cardio respiratory to adapt to a greater stimulus, then why not the CNS too?
There was a book written years ago about adapting to stress and small amounts of periods of time are good. People always seem to want a stress free life, but actually, stress can be good for you, as long as it is not overdone, just like many other things.
I think too, we relate CNS burnout with boredom in training, loss of libido, appetite mood swings etc. etc. but as far as specifics I believe it is still a study of how emotions and the mental states, relate or carryover if you will, to the physical states of our bodies.
I believe there is some research into neurons controlling specific movement, that with too much heavy or single max attempt work over time, become less responsive to stimulus and thus changing up the pattern or movement, even slightly helps or creates a different pattern. Is this why some routines use conjugate training?
As far as drugs/PEDS, I see what you guys are saying, but my question would be, that even with better recovery/strength from exogenous sources, could a level still be reached by the enhanced trainee, of a point of no return?
Drugs may enhance the length of time or total output compared to a natural trainee, but I don't think it means they would never have to deal with going past the point of no return IMO.