it´s easy if a car is registered in germany and after that it´s going to be modified. in the case it´s modified before the registration in germany it´ll be expensive to register it, because all the modified things need a sort of survey/report (don´t know the right word). for example all wheels in germany have a report of strength when you buy them. in the uk they don´t have that. so if you want to register the car with wheels without this report in germany, the guy from the tüv will say: "sorry dude, can´t register the wheels." then you have 2 opportunites: the tüv checks the strength of them (expensive) or you change the wheels.
that´s what i mean. but keep cool, it depends on the guy from the tüv. there are so many registered uk- or jspec supras in germany, there´s always a way how to manage that