Since RetroCopy is nearly 100% like the hardware, overclocking is effectively one of the only improvements that are easy to add to it. There is a reason "Disable sprite limit" and things of this nature aren't in RetroCopy like other emulators, you can't easily chop it out of the hardware and you can't easily chop it out of RetroCopy. Overclocking was possible in the hardware if you had the right tools, and it's pretty much exactly the same way in RetroCopy.

So I will probably end up adding overclocking options to all the systems as it can be fun just to see the differences.

There will be no need for such a thing for the older systems. Not as many games work better with overclocking because they aren't designed to work in that manner. Only a few game developers had the skills and foresight to design their engines such that they were clock rate independent as much as possible. Xenon 2 on the SMS stands out as one that is designed rather well in this regard, but on the actual system it runs like a dog. Most developers would tweak their engines and design such that the games would get 50 or 60fps.

There will be no need for such a thing for the older systems.