Many years ago when I was at college, we had quake1 put on the network. From that point on all we did was play quake.
From day one I was one of the best players, at the time I was only using the keyboard, no mouse. I was almost unbeatable. My friends who were using kb/m at the time could not understand how I was always winning. They had the better aiming device. I won down to a few things:
An excellent knowledge of the levels
An excellent route through the levels.
Weapon and power-up dominance
But most importantly I had years of practice playing how I played. I was as able to pull off a shot as they were and far more accurate.
At this point I looked at using the mouse, as I recognized that I was limited in my strifeing ability, the mouse would correct this. After a week or two of kb/m play I was the undisputed quake champion of the college. Completely unplayable. So much so that my friends would not play with me unless I used my left hand for the mouse, sadly for them it made little difference to the outcome.
What I am saying is you can be a great player without being an accurate player.
I don't want to speculate the level of FPS gaming I would be at if I had continued to play on the PC. But I imagine it would be quite high. Maybe if ADSL had come out a few years earlier I would have still been playing PC Fps games.
Oddly enough though I always felt there was something in the controller that would be brilliant for FPS games. I remember playing doom on the PS (I have owned that game on every format I have owned) and thinking, "You know there is something to this, there are a few improvements that could be made, but if that happens this will give the KBM a run for its money".
When Halo came out and I played it on the xbox. I knew that the controls and the controller in my hand were perfect for the console variants of this game, it more than annoys me when FPS games vary from the default halo control system.
Bit of a tangent I know. But I thought I'd say my piece.