i know the simulation is unaffected. i think i'm not doing the best job of explaining this, but let me have another go.
the simulation decides where the cars are, etc, then the game starts rendering a frame based on that. rendering this frame takes time, the longer that time is, the less responsive the game is going to feel.
when you get a framedrop, it's generally because the frames are more complicated (more cars, more effects, whatever). you don't just get less frames, but each frame displayed is further behind the simulation than running at a locked 60 fps.
it isn't just lost visual information, what you are seeing is running that teeny tiny little bit further behind the simulation than when the game is running at 60 fps.
again, this may well be impercetable, but it's absolutely there. the more image lag you have, the less responsive the game is going to feel. unless your game is running at lower framerates because of how complicated the simulations are (which i do not believe to be the case with GT5) then lower framerates = higher image lag = less responsive feeling controls.
this may be imperceivable, but it's factually accurate of me to say it's there.
and once more, for anyone not paying attention, i am absolutely not saying Forza 4 > GT5 because of framerate. i don't know which the better game is. i'm just here as part of the framerate defence force