They would not use cut scenes, they would have to render a different scene for each car, track and condition, remeber GT4 will have more wet tracks. Even if they used the game engine to render the scenebut added 3d people it's all adding to PD's problems, more to go wrong, more programming and the actual race would be interupted albeit fo a short time (100's of a second) but it could very easily cause new bugs and alot of them and there would be no way of PD checking them all. Besides it will only make people notice that they are 2d as you approach, if I'm racing in ToCa nd I pit in I see a 2d crew, that 2d crew work on the car and I leave, I don't care that tey are 2d. If I pit in and they change from 2d to 3d it would seem odd.
Finaly whats to stop PD using 3d models for the pit crew but with limited detail and a low pgc. As for only two cars racing with the top down, thats the first I've heard, I'll check that out at my mates next week.