I can confirm some changes, too. Tested the F1 karts in build 629 and I was spinning like crazy, could not complete one single lap, it felt like too much bite on the front tires. Then tried the same combination with build 590 (starting with green track both times, same plr file, same controller.ini, T500 RS+TH8RS Shifter) and it was ok. After trying some other cars I think that the change is more noticeable on the lighter cars. Heavy GT cars like the Camaro feel almost the same in both builds. Laptimes did not improve much.
Definitely no fps issue, both builds run nice and smooth with 80-100 fps.
Maybe it is just a thing to get used to, I struggled with the karts in the beginning and needed a long time to adapt to them, build 629 feels like my first steps with the karts.
Edit: another visit to Quebec in build 629 - suddenly everything was back to normal. Strange....