UPDATE 25/5/17 1:00pm: Sony has been in touch to point out additional 1080p features in Gran Turismo Sport for PS4 Pro users, specifically that along with the significantly improved in-game frame-rate and unlocked replay performance we've highlighted in the 1080p comparison further down this page, anti-aliasing is also enhanced with the inclusion of 8x multi-sampling (MSAA).
This does change the conversation, as 1080p with 8x MSAA would be a fascinating comparison point with an 1800p checkerboard presentation downscaled to full HD resolution - conceivably, the presentation could be cleaner. Polyphony has done something similar before with its lower resolution modes of course, with improved MSAA present on the 720p mode of Gran Turismo 5.
It's an aspect we should have picked up on and apologise for missing, because reviewing further media does show the difference - though in fairness, the MSAA coverage does seem a touch inconsistent and the effect overall is subtle. Polyphony Digital has supplied a brace of shots here (the first two in the sequence), and we've supplemented that with a few of our own shots.