Here's a thought about the 170 / 830 cars question which just crossed my mind. They say that there are 170 "premium" models with cockpit and fully visible damage from inside and outside, and 830 "standard" models carried over from GT4.
Now, think of this: to be able to generate fully visual damage, you need to model a car completely, with all its parts. Underbody, engine bay, everything. Only then, you can make sure every panel smashed open or lost will reveal a believable sight. On the other hand, cars in GT4 only were an outer shell with a bit of cockpit modeled in (you were able to see the driver and parts like a steering wheel, but not much else). So if you were to show damage with one of these, you would just see the inside of said empty shell when the car loses bodyparts.
My interpretation is this: the 170 refers to the cars fully modeled including all its individual components, so a fully developed damage model from inside and outside is possible. 830 cars are based on the data that was gathered for previous Gran Turismo games, so while they will have the same HD looks on the outside as their 170 counterparts, they will not feature damage, because their visual model just can't cope with showing its bowels. When PD gathered the data about these, they didn't think that one day they needed to model the car from ground up, so they simply didn't collect the necessary data. Whether those cars will feature a cockpit can't be said with absolute certainty, but I don't see anything that would prevent them from.
So in this interpretation, it is:
170 "premium" models with fully visible damage inside and out
830 "standard" models just made of an outer shell and a cockpit (= no damage)
