For the first question, the physics surely will be better than GT Sport! I doubt it will be as good as iRacing, but it's pretty sure that will be more realistic and better than GT Sport. In the videos we can notice improvements on suspension model and probably even tyre model. And they're just 1-2 minutes videos, but the difference is tangible. More content will come in the next weeks, so we will be able to see more and judge more accurately. I expect some FB improvements also.
As for the second question, I don't think PS4 and PS5 will have different physics, I suppose the major differences between PS5 and PS4 will probably be just graphics, framerate, 3D Audio, RT Support (only replays and scapes), 4K resolution, dualsense adaptive trigger and feedback, loading times, HDR (maybe) and MAYBE the VR support when PSVR2 come out. But I think the rest gameplay and content wise is the pretty much the same.