But presumably you play using an outside view ? Try switching between o/s and cockpit when on a straight at Le Sarthe, Bathurst, and parts of Belgium... usually where there are lots of trees lining the track, or on pit staights**.
Framerate issues appear to be ONLY in cockpit view, on SOME tracks, at least at 720p - I couldn't speak for 1080p as these consoles struggle anyway with that - and you'd need to be sat just 3 feet from a 50" TV to see the difference anyway !
** and in addition, if yo have Reduce Flicker set to High, the stuttering is obvious as Vert Sync is thn switched OFF. Instead yo get screen tearing.
There is a also a lot of flicker/shimmer caused by the trees, which increases the faster you are going - this lack of Transparancy Anti-aliasing, adds to the framerate issues if you are sensitive to it. The first straight at Bathurst in cockpit view with Flicker Reduction at high, "feels" like its running at less than 30 fps - go to hood-view, and it's like butter. So I guess, it's your choice.