This wasn't LIGHTNING fast, but it was good. If you want a COMPLETELY standalone system, i.e. A box with a USB port and some buttons, you will end up wanting to make it easy to Ganges out your snippits easily. A panel mounted SD card slot connected to whichever MP you go with is probably the most obvious option. The last time I did something like that, I made a dir structure on the SD card, one directory for each modifier key - essentially I had 4 modifier keys that could be pressed FIRST and in combination with my main keys. Into those dirs, I put a folder for each main key. If I were doing this myself, I'd use the teensy because its small and the HID support makes it ultimately portable on any machine. I'd then throw an LCD on it - a nice high res OLED perhaps - and have it display, at rest, a basic list of keys with a hint, then when pressed, display the text about to be inserted.
I'm just really hoping i can get this to work in a way that it can pump a 300 character long snippet of code into a pc's text editor without any lag and - most preferrably - without any software running on the pc.