Here is what I did to achieve good graphics (for NVIDIA GPU's):
In-game settings:
1- Turn off HDR and Motion Blur (both options are causing that blurry aspect);
2- Set Anisostropic Filtering to 2x (it's VERY resource intensive on this game, don't know why);
3- Set Shadows to High and AA to 4x;
4- Turn off Vertical Sync (use NVIDIA Inspector's one instead);
5- Turn off "Lock Onboard Camera to Horizon";
6- Set Cubemap Resolution to Low (very resource intensive) and Faces per Frame to 6 (for real-time car reflections);
NVIDIA Inspector:
1- Create a profile for Assetto Corsa (add both acs.exe and acShowroom.exe);
2- Set AA Transparency Supersampling to 4x Sparse Grid Supersampling (say goodbye to jaggies and ugly shadows);
3- Set Vertical Sync Tear Control to Adaptive or On (optional).
Results: 60 FPS all the time (except during Monza replays).
My configuration: i5 2300, 4GB, SSD and GTX 660Ti.
Screenshots:
http://imageshack.com/a/img824/434/1wza.png
http://imageshack.com/a/img706/8109/7bka.png
http://imageshack.com/a/img41/2555/xpqs.png
http://imageshack.com/a/img853/9407/ksy6.png
http://imageshack.com/a/img29/3084/8nz7.png