I've owned both, and I kept the FD2. Mumphrey is pretty close, but the KOT can easily have as much gain as the FD2- all you have to do is stack the 2 channels. Basically, the 2 differences are:
1) FD2 is an overdrive with an "additional gain stage".
KOT is TWO overdrives in 1 box, each side can be used independently, or they can be used together. Each channel has it's own gain, vol, and tone controls. It's the PERFECT overdrive box, design-wise.
2) the FD2 is more compressed, smoother, and has a slightly different midrange signature. I personally prefer the TONE of the FD2, but it's just a personal preference. I WISH the FD2 was made like the KOT tho....
in other words, if the KOT sounded like the FD2, it would be MY "holy grail overdrive". It'd be like 2 FD2s in 1 box.
"I've got callouses, from all those nights, spent playin' a Telecaster, 'till my fingers bled Bud Light" - Travis Tritt