I've made a really peculiar discovery...
After playing around with the course maker for a bit, making jump tracks and such, I noticed the AI never seemed to roll over. I wasn't sure if it was because the AI were being extra cautious or smart when handling odd terrain, or if I just wasn't paying attention, but I started to look into it a bit more, and found that the AI cars seem to be opporating under different rules than the player car. Perhaps the biggest item is that they can't pitch or roll beyond 90 degrees, similar to how your car behaves online. On bigger jumps it also looked like the cars would "correct" themselves in mid air, and sometimes lose inertia and sort of "drop" out of the air in an odd-looking manner.
I wondered of this was exclusive to course maker tracks, perhaps as a last-ditch effort to conserve processor power (I couldn't really think of another reason considering you have access to the "full" engine), so I went to Daytona to cause chaos, slamming them into walls, and hitting them head-on on the banking while going the wrong way. The results were similar.
I'm working on a way to make GIFs of my findings, as I don't really want to put all the footage on my YouTube channel... I recommend trying it yourself on the mean time.
I don't know how far the differences extend, but I find it really odd that a difference exists in the first place. I know it wasn't there in GT5. It's a bit disappointing for me, as I like playing around with the physics and other various tools that game engines provide, but it seems PD's limited certain aspects.
*Edit: Another thought - I don't know if this discrepancy in physics has existed in every version of GT6 since it's launch, but it could be a reason why the hidden alternate routes of Route X were canceled/held back.