Well, you are aware of the indications that some cars in Forza take more damage than others, aren't you? I noticed this myself, but assumed it was just Microsoft-itis rearing it's buggy head. Evidently MS could only afford so much, and had to settle for compromises.
Kazunori-dono wouldn't settle for that, I'm pretty sure, and it would be scandalous if it was implemented and discovered in a GT game. Forza is another story, as people forgive MS the equivalent of genocide.
Keep in mind that Sony Computer Entertainment is spending tons of money on many projects, from buying studios to funding them, supporting others to make sure their games are top notch, as well as making sure the investment in the PS3 and Blu-ray is covered. They may not be able to afford two or three or more king's ransoms to acquire the rights to Porsche, Lamborghini, Maserati etc, especially with MS themselves throwing crazy money around like a certain president.
And stating that Kazunori might not want a car or track included in a GT game, that doesn't sound like Kaz at all. This is the guy who insisted that dinky 60hp Suzukis were included in GT4. I'm sure he'd want every car from history, including prototypes if he could manage it.
And this is how I see it too. Kaz had a dream to make Gran Turismo way back in the beginning like some kind of alternate life involving cars and motorsports, but he couldn't realize this on the more primitive PS1 and 2 hardware. Well, the PS3 has the power, rivaling many gaming PCs. While the physics has the distinct Gran Turismo gene, which some of you pointed out, it's still very advanced and revealing. I wouldn't be too surprised if some car makers wouldn't like their cars being in an environment where they could be compared freely with a number of other high performance makes. Especially direct rivals, and especially much cheaper cars.
I'm hoping this isn't the case, that these car makers are grown up about this, *cough* and relish the chance like BMW in J Clarkson's remarks above to be in the most famous racing game ever. That "by golly, if Ferrari is in there, we damn well will be too!" Work with Kaz and the lads and be a part of the family, and let us enjoy owning your cars too.