This debate has been overdone time and time again, especially in the training subforums. Enziel isn't completely wrong per say, I just don't think he's explaining it right.
When it comes to power/speed, I believe genetics are the key factor in determining our ceiling, and we can only maximize the individual potential given to us up to a certain extent. As others said, maximizing your physique through various types of training, decreasing fat, and perfecting technique can all maximize one's base potential, but once that individual peak is met, the effect of those efforts begins to plateau. As most fighters have already put in those maximized efforts, I can confidently say no fighter has ever been able to become quicker than they once were, especially past their physical prime, not unless they were like Crazyhorse who never trained hard to begin with. The only thing that happens with increased age is a decrease in speed/agility, as it is one of the first things to go.
We'd all like to believe we as humans can do whatever we put our minds too, but that's just a lie to mask our insecurities. It's like when you see people who try and pick up instruments but have zero sense of timing. Yeah they can get a little better, but ultimately they will never become a Mozart. Same shit applied to boxing, I see it in my gym all the time. Look at Keith Jardine. No matter how hard he trains, he will always have that hirky jerky style, and will never look as fluid as Mayweather. Same shit applies to speed. Sure Mir can become a little faster, but he will never match Lesnar in speed despite being smaller. At least that's what I believe.
Sorry for the TL;DR lol.