I've gotten a few more hours in. On an i7 6700 w/ a GTX 1070 and 16 gb of DDR4, all settings on Ultra with a forced 8X MSAA/4X alphas at 1080p, both expansions are all but perfect at locked 60 fps with Nvidia's "Fast" v-sync setting as the override. With an unlocked framerate, I see high framerates with noticeable stutter (80-120 fps, in normal conditions).
The main game is totally solid everywhere but one end of downtown, which is still veeery playable, but I pick up on frame-pacing flicks every few minutes of driving around. I see this in a lot of PC games, and I don't love it, but it's subtle enough to tune out.
The improvements are solid, but I'm sure this would be annoying on a lower-end system, if those issues scale to spec. Also,
here's a got'danged dinosaur eating the track at 80 fps