They might change it even if it's a bug or intentional. Remember this type of game is new to Rockstar Games so they are going to change and tweak a lot before it's even completely finished. They released GTA Online and will be tweaking based on Community feedback given to them.
I think they take some customizations off. I know Turbo stays because you can hear it but I don't know if anything else goes into races. If you're using a custom car, they want to give you a slight advantage but not too much.