Hmmm, after fiddling with the game for a while, I've found that the only sure fire way to completely eliminate all slowdown in the game is to run at 1280x720. Yep, that sounds rough, but when viewed on a 50" Kuro from 7ft away it's plenty clean (though it looks terrible when viewed from close proximity on my higher resolution 27" PC monitor).
At 1280x720 I get 99.9% consistent 60 fps even in all of the trouble areas. This at least allows all of the details to be turned back up. In some shots, turning things all the way down doesn't seem to have a huge impact, but in reality, there are a lot of things that go missing. Reflections were a huge one. Anything other than high eliminates the awesome road reflections.
If I drop some settings down (including reflections) I can use 1080p with mostly solid 60 fps. Turning those back up results in dips in a number of areas that ultimately winds up ruining fluidity. It's pretty annoying, no doubt.