Remember the whole name of the simulator:
"LIVE FOR SPEED: Online Racing Simulator"
Besides IMHO the IA to play off-line just sucks, If you slow down because whatever happened to you, the car behind you just crash against you back you no matter the distance/velocity, they simple are dumb cars around a circuit. And that's fair because the name of the game states clearly the online game's purpose.
Bottom line: allow mods for off-line is just useless, no real "racing" experience.
Allow mods for online is a terrible and a uncontrollable mistake. And Mods that change the physics would be even worse, that could become as racing a V8 (F08) against a Sauber (BF1), so against no real "racing".
In fact I think the Physics is this game is superb (this is believed by an AMInstP).
The way you can change the car performance via the setups is already challenging enough to spend a while around the car and then test and find the setup that best suits you.