The input lag isn't horrendously bad (though, it must come down to the TV). For one, you can barely see/feel any difference at all when you're playing a 60fps game. Give Rage a try and switch between. The input lag varies by game. BF3 on 360 isn't off too much to cause problems, but Gears Judgment felt a tad off and felt "floaty."
Amount of fps is irrelevant, as MotionPlus crap essentially just buffers each image at its refresh rate. If it's a 120Hz TV, it'll poll the frame every 1/60th of a second (60Hz). The input lag will be with
perfect, infinite computing (sounds like a job for
the cloud!) at least one frame, since it holds back the first frame it polls, then gets a second. Then it interpolates the two by seeing what changes between the two images and then assuming very much on that movement. Things like crosshairs and HUDs will be a problem, since they're a non-moving factor on a moving background, and the TV is like "I'm assuming the HUD will move with everything else".
So yeah, 16,6ms lag MINIMUM. Notwithstanding the computation that has to be done on the two images it's holding back to create the interpolation. I'm guessing minimum extra lag created by this is in best case 50ms, maybe 100ms is typical. I have one PC screen that has about 8ms lag and one that has about 50. When I used to use the 50ms one, I thought it was perfectly fine. Now I'm used to my new screen, and when I move the mouse over to the other, it's like "holy crap, my mouse is entrapped in syrup". You can get used to the feel of input lag, but it doesn't mean you've basically tripled your reaction time in whatever game you're playing. Your normal reaction time is basically 25ms. Throw an extra 50ms on that, and you're up to 75ms instead. You're just putting yourself at triple the disadvantage most other players have.
All in all, never ever ever use MotionPlus for anything game related. If you enjoy artifacts and interpolating movies, go nuts, and I'll shake my head while you're not looking, but at least you're just ruining the movie, not creating input lag for yourself.
Really, just never use it.
EDIT: Oh, and turn off all your other 'auto' crap too. If you use shit like dynamic contrast and adaptive coloring our whatever all that shit is called, your TV is holding back everything even more. Turn it off. Right now.