Hi Mike, I had the the same car and diffuser. I had the standard Cupra R exhaust fitted but the diameter of the pipe on the Cupra R is larger than the TDI. So I had to have my Cupra R back box pipe adapted to fit. I'm sorry I forget the sizes but it was something like 2.75" vs 2.5" maybe.Not massive.
If you're getting a full system you may need to check the difference in the connection of the cat back of the TDI vs the Cupra R. The Cupra R system is likely to be bigger further forward too, so it'll need some kind of adaptor to create a step in the system.
Also I found the Cupra R back box just as quiet as the standard TDI back box, so look at getting a loud Cupra R system too.
Only pictures I have lol
Oh and Scorpion over Miltek