I know another couple of folk local with Cupras. Will compare mine to theirs when I see them next just to make sure.
I just thought it was weird that mine moved so much more than the engine in the older, higher mileage MK5 Golf GTi
That sounds like an engine mount-more likely that of the passenger side as you describe-has become loose...
Have you done a lot of launching the car from start and in the dragstrip...???