You're right about the US cars. If I can find again that post on FB from the people that did it and asked questions (don't count on it, almost impossible!) , then I'll see which lights they had before. Pretty sure one of them was a Clubsport that did the conversion from stock Euro LED though...
As a side comment, I thought about this briefly a while ago but a) too much money for something I don't ever get to see even when looking at the car from the outside b) think it's going to look a bit weird without the fronts also and c) I get the appeal on standard GTI cars, as there is a 7GTI and a 7.5GTI so you're half upgrading to that, but there's no 7.5Clubsport. Just my personal opinion of-course.