I’m just curious why you are passing on the Pi. I set one up a few weeks ago using RetroPie, and it works perfectly. Since I had everything I needed for the system except for the Pi, itself, and a wifi adapter for initial setup, the whole project cost me less than $50. You can’t beat that.
I’ve only run Nintendo and Super Nintendo games, dozens of them, and I’ve yet to run into a single problem. The only challenging part was getting my Xbox controller buttons configured to work well on both NES and SNES, since Nintendo moved the positions of the A and B buttons somewhat between those systems.
I’ve played FX games on it with no problem, and I can’t imagine Genesis games would require overclocking, either. I’ve never once experienced speed related issues with any game. They play seamlessly, at least all of the ROMs I’ve tried.
My guess is that if people are having problems, they’re probably introducing them by needlessly tinkering. More advanced systems than these might require overclocking, but these do not.