I doubt this will be implemented before release but I think that players should only be able to customise the look of their car, and not anything that would give them an advantage over other players. This is because if you win a race and get nitro, it becomes a lot easier to win future races. This means that you'll keep on winning and everyone else will keep on losing which isn't really fair.
They check for unusually high speeds at the moment. However, you need the SDK in order to mod Blue - it's not as simple as editing a file. The SDK will most likely be released after Deathmatch.