Ok, here's my thought. I have a lot of co-driver experiences but first hand: I have driven a race-prepped Ford Sierra Cosworth (RWD version), Mini Challenge cupcar and a VW funcup beetle on both Spa Francorchamps and Zolder in Belgium. I do think you can actually take some of the sim abilities to the track, but experience wise you cannot and may not compare.
As a co-driver the thing that you really notice (especially if your light-weight like me) is that you have the feeling of being thrown around. When you're driving you have the anticipation of what the car is going to do because you tell it to, but as a co-driver you do not have that luxury and you feel all the forces working at your body.
The main thing you miss obviously is speed sensation and gforces. When playing GT5 I "feel" when the back steps out but I think I "feel" that because of an abnormality in pixel-direction (?), the image I am looking at does not move the way I expect it to (sounds weird but cannot explain it myself).
In a real car you feel when the back steps out with your behind in the seat and you can feel it a lot soon than you'd expect.
My weekend-car is a 944 and 3 years ago it was my only car, going to work in the snow, even then with that car the rear axle talks to your behind through the seat. "Watch it, lad. I'm about to break traction here! Wait for it ..... wow, nice catch, almost lost it there!"
Maybe this sounds like incoherent mumbling but I cannot explain it any other way but it comes down to this: a wheel cannot take the place of your behind, FFB emulates it together with the image you're seeing so you can have some indication but not a realistic one.
Yet another thing, the funcup was without servo-steering, as a lot of (race)cars are, and -my god- it feels like your arms are about to explode after 7 lap francorchamps something i haven't felt in GT5 on that very track. Force feedback cannot replicate that experience as force feedback is also used as a tool to converse to a gamer what the car is doing. This because gamers only have limited 3 way-communication, being hearing (quite realistic), seeing (only the screen), and feeling (only FFB).
Braking is another thing you cannot comprehend if you step into a racecar. You think you have to brake about here to make the corner. No, brake even further, further, even more further. Something the realy steering tells you but FFB doesn't. Neck-snapping, you wouldn't believe.
On the upside, as I already said, I do think that you can take the basic understanding of racing, track knowledge and the enhanced reaction times learned for racegames to the track as an advantage. But that being said, I think it's not fair to make a definitive comparison between the two. (unless weare talking about moving 3-axis simulators, but there,I lack any experience)
TLDR: simulations can help but they're still "simulations" of real driving/racing and FFB is even more simulated.
BTW: I own a G25 and I like it, no complaints, but see TLDR ;-)
