Set car reflections to low and they will look better. The lower detail will make them smoother instead of being a bit more highly detailed but very pixelated. And because they are a bit lower detail and a bit more blurry, you wont notice the update rate of the reflections. I dont see this changing though. Reflections are seriously demanding and this is an old game on an old graphics engine. It just isnt worth the big hit to performance to make them purty.
Should still be optional in my opinion. I get 250 something FPS at home. I can sacrifice some of those for smooth reflections. Developers better listen up!
I noticed it yesterday by driving open wheels and P2 Radical. I really hope to see it fixed, it's quite ugly otherwise and it brakes the immersion a lot. I changed to "low" the reflections on cars, the lag is not so huge but it's looking so bad...