Mechanical keyboards are the end-all be-all when it comes to KRO, as you can push all the keys you want at the same time, and your PC will register them.
Example = da - I just pressed 8 keys and got "da" because I am on my work keyboard. If I was on my Filco at home you would have seen asdfjkl;. Another function of mechanical keyboards is that they create a hardware interrupt when your press keys, so they keystrokes ALWAYS register.
Mechanical keyboards are expensive, but they can handle millions of keystrokes, and are far superior to membrane technologies.
If you do go the mechanical route - try before you buy. The keys feel very different and the choice between blue, red, brown, black, etc makes a big difference in feel and functionality (i.e. where the key engages).
For what it's worth I really like browns. But the next guy will like blues or reds. I think most people don't like the black keys because you can't tell when your keystroke registers (there is no actuation sound or feel) - but the black keys are typically advertised as the "FPS" keys - they may be perfect for you..