Hi there, roby13.
It's not a simple answer, I fear.
The AI of GSC with default content (rcd, plr, physics files, terrain and AIW's) is the closest I have ever seen to Barry's rework of AI & track data for GTR2. Anyone mildly familiar with Reborn should know this is a true compliment: I like GSC's AI a lot.
On the other hand, with different, non-default tracks comes different terrain and AIW files and things inevitably change - unless the modders that created those tracks are "AI-aware" and were careful about it.
On top of it, with different car mods come different physics calibrations and rcd files. Very few modders are at a proficiency level comparable to the guys from the days of RSC (I'm thinking Bristow et al) , and even less are able to produce good AI for their own physics.
So: outside of the original content from Reiza, the work done on the AI could be better (for all the reasons hinted at and related to physics, rcd files, plr, terrain and aiw files).
One thing though: on the side of graphics and sounds, both Reiza and several modders have pushed the envelope of IM2. And that, to me, matters a lot.