Difficult question to answer, as Polyphony don't seem to provide any detail on what goes on in their physics engine, so we can only really have opinions on the 'feel' (which is very subjective), and compare some performance figures for certain cars if they are relevant. There has been some discussion in this thread about limitations of the tyre model, some suspect ABS behaviour, etc etc. The more you try to dig into the specifics, the more inconsistency is uncovered. I think it's fair to say that with such a high number of cars, the accuracy is going to suffer compared to a sim that is able to concentrate on just a few cars. It is likely that many of the cars in GT6 are using a lot of estimated data, particularly for older cars where data is either entirely missing or very limited, whereas the best PC sims use unique suspension models for every car (no doubt the 'fudging' of data happens in PC sims too, but it would just be there to a larger extent in GT due to the sheer number of cars).
There has rarely been a PC sim that can be directly compared to GT, as their focuses are different - PC sims typically concentrate on race cars, whereas GT is mostly road cars. But when there is something to compare, the PC sim tends to impress more of the time, imo. As Metalmurphy mentioned, GT Legends captured that classic car feel a few years ago, and more recently that was given an overhaul in the shape of P&G, which tightened up the accuracy of Simbin's effort over several years, with an attention to detail that few can match. The road cars in Game Stock Car also feel more nuanced than what you get in GT, the karts feel more like karts than GT, etc. Niels' recent rFactor NSX release also feels more realistic than the NSXs in GT6. But this is all on the same ISI engine, which is continually proving to be one of the most advanced in physics. Other engines like iRacing, LFS, RBR all have their strengths and weaknesses, and I believe they also beat GT6 at certain things (e.g. RBR's loose-surface behaviour feels better than GT6's rally, iRacing has cleaner FFB, etc). Recently Assetto Corsa has offered a few cars that can be directly compared to the ones in GT6, and it also appears to come out on top. Again this is subjective, but the cars I drive most in AC, the M3, 458 and F40, all appear to have more believable characteristics than the same cars in GT6.
There is probably just as much inconsistency in the huge mess of PC sims available as there is in GT6 due to incorrect data or incorrect application of data. The difference is that a lot is known about PC sim engines, but hardly anything is known about GT's engine. There are specific examples of where PC sims are more accurate, but in terms of overall feel, GT6 does an impressive job of reproducing mostly decent behaviour on a huge number of cars. Going on what I can gauge from feel, I don't believe it has the depth or accuracy in its physics as the best PC sims, but if Polyphony were to be a bit more open about what their engine is doing, I'd be happily proved wrong.
