This mod was made and tested on the Physics Fine-Tuning MOD FINAL version but it should work on other mods with similar physics with minor adjustments.
Changed/fixed AI lines at various places. As editing each track takes a lot of time and effort I decided to update this mod when I feel I have satisfactory results so bear with me. Latest update contains only edited splines for some tracks for the Moto2 and Moto3 class as I edited them while I was playing trough the career mode in the game. Next update will be of all the tracks for the MotoGP class as well as some tracks for the other two classes. This is still work in progress so any feedback in the form of TRACK/TURN make it wider or tighter or I don't like this turn or something else will be greatly appreciated as well as how it works with the physics mod that you tested it with.
Installation: Extract the zip file. Use MixFileRemixer to extract the DATA.MIX file (make a backup in case you want to revert to default), then in the TRACKS folder find the track you want to edit and just copy and overwrite the correct SPLINELAYER.bml file and OPTIMIZATION bml file. After that go to the DATA\VEHICLES\AI folder and replace the AI_DIFFICULTIES.BML file for the track that you are editing. When you are done just create the new DATA.MIX file.
Important: Create a different DATA.MIX file for each tracks and class. For example Moto3ArgentinaData mix or Moto2AssenData mix and replace them in your game directory as you play, based on the track that you are playing on. This is important because of the AI_DIFFICULTIES file and because the AI lines are different for the moto2 and moto3 classes. I couldn't find a solution for one optimal line for all classes sorry. Not doing this will probably results in AI crashes or AI cutting corners.
Note : It is recommended to use this mod on highest difficulty and with pro physics. Also for added difficulty turn off the TCS if you like, and remember this is still a game and a crappy one at best if it wasn't for all the mods here it wouldn't be playable. And it can be as realistic as you make it. For example try passing cleanly not just bumping into other riders and forcing them wide, also when starting a race try timing your throttle with the lights in order to get the perfect start and have fun. Also the tutorial has been updated for editing the optimization.bml file.
As requested I've made a tutorial on how to edit AI lines. You can download it here - https://www.dropbox.com/s/ihtwio7pvatwacn/AIGuidence editing mod tutorial.docx?dl=0
Finally a big THANK YOU to Dyego JhOu for adding some functions to the BML editor and making my life easier.
Here are some images: