This is another problem in having factual debate, people can find what they want to prove their point. I.e. 'this game is accurate because car x does this like real life' where another can say 'no it's not accurate because car x doesn't do that.' This is for any racing game. Unfortunately no game has got it 100% right so far so there are always things to point out as being right or wrong.
There was a big long interview with Dan Greenawalt in autosimsport magazine around the time of Forza 3's release where he mentioned something that stuck with me: it's one thing to simulate the car accurately, and it's another to simulate it to people's expectations. So if people expect your Veyron to have 1001HP, it doesn't matter if it actually produces 998HP in real life - people are going to hit you for it if you don't have it doing what "everyone knows" it should do.
Back when Shift was released there were some really weird arguments that were - paraphrasing very little here - seriously along the lines of "
lamborghini's don't slide". Or "
mercedes don't drift". And at some point that means that in that person's head, the way the car is typically driven, or promoted, or whatever, has overtaken basic facts of vehicle dynamics. So for instance, if you very rarely see professional FIA GT drivers countersteering or oversteering, according to this philosophy, that must mean that the car doesn't do that - not that the drivers are really, really good at driving and keep the car out of situations where it does so.
The other thing he mentioned is that there is a lot in reading people's intent - so regardless of what their input would actually do, if you can reasonably interpret what the driver was thinking, it should do what they wanted it to do, not what it would actually do (eg tapping to maximum steering lock possible 3 times in rapid succession should take someone smoothly around a corner, rather than totally unbalance the car). So if your game is really doing what someone's real input would do, and that doesn't match their expectations ("I'm a better driver than this" / "if the real car drove like that everyone would die"), again, you lose, because even if you're matching reality, you're not matching the mental image someone has of what should happen.
An interesting case here is Shift 2. Because Shift 2 was also released on PC, modders have been through all the files and know what the game does, and doesn't simulate properly. Even with all the evidence of what is being simulated well, I still see many arguments about the physics being terrible etc (misunderstanding steering lag/ffb issues for a bad physics engine). The devs only softened the tyre grip dropoff around 5% to assit pad users.
We don't have that luxury with Forza and GT, so it all remains in the realm of speculation and what we can test within the game, which can often be presented to back up any view, good or bad.
One thing seems obvious to me, GT5 struggles to simulate tyres/suspension, Forza 3 does a fairly decent job of tyres but also struggles with suspension. (Shift 2 does suspension really well, watch a replay going over bumps/curbs/jumps, it makes the others look instantly very basic)
It's a good system, but it has its weaknesses too. For cars where the suspension system is very specific (carterham, 997, etc ) it does a really good job. Some other cars and layouts, it's not so good and quite generic in places. It might not seem intuitive but at a certain point, when you add loads of parameters for data, it actually becomes quite hard to sensibly fill it out. So for example, if you have an engine which supports ~60 suspension setup parameters per wheel, and make it interact with a 6DOF suspension model, with every component having its own dimensions, 3d inertia and mass properties, that is actually a pretty scary amount of data to come up with for even one car, let alone a hundred (and not forgetting you then need several upgrade levels, and interact with loads of surface types under loads of conditions). So at a certain point you have to cull things down and simplify it so that you don't go mad, or, at the lowest level of fallout, end up with certain cars behaving radically differently with certain upgrades installed.
In Forza 3 for some reason my cars have some "soul" to them. Seems like they have this little something they do and others don't. In GT5 cars seem cold. Like a collection of parts that operate in some clinical environment that doesn't represent the real world. GT5 just lacked soul to me.
but many still look great. And I think Forza's cars are modeled very well already. It's the lighting that was off and seems to have been redone for FM4. 
