I agree, most of the top drivers in the world wide ranking display the basic fine motor skill and brains (to find out what it takes to go quickly) to become a very talented driver behind the wheel of a real race car. Now not all the top drivers in GT will become successful in a real car, but chances are, the top guys would be quicker in a real car by a fair margin than someone who is 2 seconds off pace in GT, as long as they have the guts and confidence to push a real car to the limit.
I also believe there is something a bit off with the dynamics of the Tuned Z and what it takes to go fast when in comparison to reality. But IMO, the stock Z follows the rules of real physics quite well. The only real difference between a real, bone stock 370Z vs. the one in game is the fact that the stock Z in the demo lacks a lot of traction (mostly in lateral grip). The basic physics aren't really all that abnormal, it's just the fact that the car display a general lack of traction, which could basically replicate running on a complete crap tire. Now this doesn't change the fact that the physics mirror reality quite well with this car though. More importantly, the physics aren't off enough to account for the 2 second loss in lap time that these self proclaimed real race car drivers are missing.
I think it's also worth noting that the top lap times that we see in GT are generally less practical/realistic than what you would see in real life for a number of reasons. I would realistically say that the top lap times we see in the demo are probably .05% (~ .5 of a second) faster, or closer to the ABSOLUTE limit of the car and track than what you see in most real life GT (Super GT, FIA GT, etc.) type racing qualifiers, where the top qualifying laps are not nearly as accurate and at the absolute limit compared to a top lap we're used to in Gran Turismo. So an awesome lap in REAL GT racing would look far less stellar than what we are used to in Gran Turismo for a number of reasons - the Gran Turismo driver has logged an endless number of laps, there's is no concern for crashing, conditions are much more stable, etc.