Silly? Surely a driving simulator should be trying to get as close to real life as possible. Ergo an improvement in the simulation should result in a decrease in the delta between real-world lap times and in-game lap times. Thus if GT1 had Car X getting around the Ring in 4 minutes where in real life Car Z would take 8 minutes, and GT2 shows the car taking 5 minutes, GT3 6 minutes, and GT4 7 minutes, you'd see the times getting closer as a result of the simulation moving from a very weak arcade racer to a more accurate simulation (yes, very coarse, unrealistic figures just to illustrate the trends!). If GT5's simulation engine is more sophisiticated that predecessoers, those time advantages caused by not factoring in real-life limitations should be reduced, meaning closer-to-real results.
That's all of course, no brainer, taking all things as equal and accounting for varables. A change in variables like tyres or weight of the driver will add variation to the gam eresults and real-world result. But as others say, these variables aren't going to have a huge difference. the same car on the same track with the same driver isn't going to see a 10% improvement from chaning frmo one brand of slicks to another, unless the intial brand are extremely sucky tyres! A windy day isn't going to add a minute to your lap times on the Ring. So we'd just be looking at same ball-park race times for an accurate simulation, and not exactly the same times. Where in one test of GT4, a Ford GT managed Leguna Seca in 1:38 in game best time, and 1:54 in real life, that difference of 16 seconds (16%) isn't going to be explained by a different brand of tyres and a little more headwind, but a difference between the game and real life. The question is whether the core simulation isn't very accurate, allowing cars to perform better in the game than in RL, or if it's a matter of the drivers.
The ultimate simulation game would have players getting about the same best times, give or take a few percent difference, because the cars' handling and the racing conditions reproduce the physical state of the cars and the mental state of the driver. We should be seeing some more realistic times in GT5 versus GT4 for the same car+track combinations if the simulation has improved.