There's a bunch of liveries and the tuning is real deep but theres no visual mods to the cars. i play from helmet view anyway and the game lets you focus on the actual racing instead of what rims you have and what decals etc.
and no you cant add any performance mods. all cars are already modded to be at their best. you just need to tune them.
edit: it plays completely different than forza and gt. i read someone elses comment somewhere that said pcars makes gran turismo feel like cruisin usa. i agree. same with forza.