Both in terms of power and functionality. With what we currently know about materials science, how much power in joules or watts could a humanoid frame with densely packed artificial muscle fibers like a gorilla or a chimp's exert? How much of the human body sans brain could theoretically be replaced with artificial muscles and organs?
Find out the tensile strength of the strongest material (e.g. carbon nanotubes) and apply the cross section area of the muscle. But if using supermaterials you can go beyond the limits of atoms, so there is no limit other than the amount of energy you have to apply.
Depends on the efficiency of construction.
I would say 99% efficiency maybe possibly, and temperature like 1200K is possible(maybe more 2000K). So something like 15000 HP in human size body is possible with artificial muscles, systems, if not as constant then peak. Peak values could be even more especially with 100GPa strength materials like carbon nanotubes.
With more conservative and save estimation of efficiency, like 30%, we might expect 5000HP. So car powers have to be expected in worst cases.
Just estimate by efficiency and energy dissipation and working temperature - that's fine.
There are some examples of materials in A from Q human-like robot muscles
More a problem will be the energy source than the ability to use it.
Also, I do not see some special need in replacing parts of the human body to squeeze maximum, exoskeleton will work the same way and it will make possible to apply greater power-forces and have more energy in possession and allowing to have more bulky units to generate it(thermonuclear reactor as an example). Just reinforce human body a bit and add exoskeleton that's all is needed.
And yes forget about metal, and how most machines made today, no comparison with them, this way will go into history as steam era did.