Thanks guys. Based on the 3 GB video, I believe it is a native 1080p. Antialiasing seems to be effective only in replays though.
I did not complain about compression artifacts, but the quality of engine rendering. Here you have uncrompressed PNG from original JPEG, enlarged to the same size, but with bicubic interpolation. It is still clear that the picture has reserves regarding quality. If you have large Full HD TV, you will definitely notice that.
edit: fixed image size
In overall, I'm pretty disappointed with GTS visuals. I would not believe in 2010 that I will say that, but I can't help myself: Forza 6 looks the same or maybe better in some terms. At least, GTS looks inconsistent at the moment and I'm skeptical it will get better on the same hardware (PS4K finally could make it look sweet?). Nonetheless, the way pCARS render bodyworks on PC is even one level further and no Sony magicians, even in Polyphony Digital, can beat that. Where are the days of GT5P?