You can still upgrade the car after you race modded it ...
Should be able to put a supercharger or something on it...
However, the problem you have to realise is an RM car is not the same as the REAL version. The real version has a different build/engine etc, so you cant expect the RM version to be able to keep up with it.
He will have to equip the parts again, if he had them on before Race Modding it.
Could be that he is using the metric system. 6xx kw is equal 9xx hp...
upgrade the engine. You'll end up pushing 950hp...