The Pi Zero is a tiny bit faster than the original Pi, but you cannot over-clock it. It’s a great little unit though. The extra RAM makes it more useful than the model A/A+. I have used 2 already and are awaiting stocks to get my hands on another. I will try a Pi2 in my next model which has quite a bit more grunt.
I have left the display exposed as I want to get the touchscreen working. That way I can select the emulator and game using a stylus. I have a printed and laminated surround to tidy the screen up.
Those Gameboy cases are nice, thanks, but obviously you don’t get the circuit boards. I modified the original front circuit board for the button contacts and used the LCD contrast control for a volume control. I also used the original speaker. On the next one I will probably use the headphone socket PCB also.