Thanks wetriner! I was able to fix the updated version of Mupen64Plus.

I deleted my mupen64plus.cfg like you said, and it regenerated the file so that I could fix the hotkeys by following these instructions:
http://blog.petrockblock.com/forums/topic/8bitdo-nes30-pro-hotkey-problems-in-mupen64plus/

I also was able to find the new location of the save files at /home/pi/.local/share/mupen64plus/save/ whereas before it was storing the save files in the roms folder.

Whatever is submitted to the RetroPie-Setup repois updated.

@goatsoup the documentation is useful...

https://github.com/retropie/retropie-setup/wiki/ZX-Spectrum

Also, disable the high resolution mode in the lr-pcsx-rearmed core options. It's a huge hit on performance.

Hi BuZz,

Thanks for the reply. I believe it is a Retroarch issue in both cases on certain cores, as using a keyboard directly plugged into the Pi (removing the Picade from the equation) suffers the same response and mapping issue. You are correct, I think that experimental firmware is for the retail boards, although in the video posted in the thread, he does mention it'd be pretty easy to make the same firmware available for the kickstarter version, just a few changes are needed.

I agree with the Amiga emulator, I only tested it quickly but when I realised I couldn't even navigate the menu without a keyboard, I moved on for now. I am trying to keep my Picade setup as compact as poss and everything else is working great. I'll revisit it later, I hope one day it is ported into Libretro or developed further. Some games run really well.