I made a thread a while back about trying a Panasonic Plasma G10, and a Samsung B650 LCD in COD4. From my experience at the store, the B650 even when not in game mode seemed pretty responsive, and controls did improve slightly with game mode on. The biggest difference between the 2 panel types was the amount of ghosting/trailing. I noticed when panning around on the LCD, there was alot of blurring on screen. The G10 blew the LCD out of the water in this regard.
So from that experience, since I am not a pro gamer (maybe intermediate-advanced), i would say that input lag from a console seemed overrated, especially with game mode on. The blurring of the lcd image was the biggest turn off.
As said before, input lag (and ghosting) will vary from lcd to lcd and plasma to plasma, but everything ive seen and read points me towards FPS on a plasma as the way to go.