I will agree that it sucks there is no mod shop, but i think it would take A LOT of time to get the physics right for new cars if there were bodykits that were longer or w.e and new shocks etc. similar to how they decided to keep Niko's body the same as they got the physics JUST RIGHT. Also, you probably won't need to mod your car as they already are lookin pretty sexy, and I think they will be faster now so the need for NOS isnt required.
It would be realistic to have a mod shop in the game since you can in real life, but its also realistic for your body weight/muscles to change, but they had a very important technical issue to deal with, so they chucked that idea making me realize that its worth having the physics be amazing if it means no modding your car. And guys, dont worry the fact that its the greatest game of 08' will make you forget about car modding as there is so much else going on in game.