Getting my wheel setup together has prompted me to spend more time with PCARS3, getting more familiar with it, so I now feel I have a better idea of how it differs from its predecessor in terms of physics. The FFB helps in this case (not typically necessary, IMO), providing feedback that is lacking with the unconfigurable cockpit camera. Until now, I felt I had not gotten enough out of the game or spent enough time with it to make a final judgment.
I have to say that
@Scaff is pretty much right on the money. Broadly, the way I would characterize it is that SMS have "Forza Horizon'd" the tire model -- PCARS3 versus PCARS2 reminds me of FH1 versus FM4. In other words, it is mostly the same engine, but with exaggerated grip (and the omitted details we all know that I don't have to repeat).
The biggest change in both cases (PCARS3 and FH1) are the severely shortened braking distances. You can
dive hard into corners in this game. The difference between this and FH1 is that Horizon takes place on roads; Playground Games predicted that an overwhelming majority of players would not be accustomed to realistic braking distances at high speed in an open world driving game, and they're right. I never loved it, but it makes sense. In Project CARS, though?? I don't think SMS is doing anyone any favors with these wrong braking points on so many real life circuits. It's a poor fit.
The next biggest effect, at least to me, is on drift initiation, maintenance, and control. There isn't a lot to say -- tires provide more grip now. Going back-to-back on drift laps in the GT86 at Mojave Sidewinder, the GT86 in PCARS2 can drift more of the track's long bends than in PCARS3, and is more able to maintain wheelspin. It doesn't hurt that the tires heat up before long, decreasing traction. Fortunately, the effect on drift trajectory in PCARS3 is more or less proportional to the increase in traction.
I suppose it's about what I assumed it would be, once we learned about the tires. Two steps backwards, but still a solid sim -- if only Forza Horizon drove like this!