Firstly moddeling Grand Theft Auto IV is illegal as you do break the EULA (Game License).
You are not allowed to change the games engine, therefore modding it is actually not allowed.
On other hand changing and kind of handlings is hacking (it could give you advantage).
Faster cars, Better handling - Rules clearly says it's not allowed therefore you will be punished.
So regarding that 2 facts, it's your decision if you really do mod it or not ;P