The site says that the kit consists of the air duct, filter and CF airbox and costs 3500EUR

The radiator does look aftermarket though (silver etc) and it does seem to be lower as you said.
As far as I recall I don't have a fan in front of my radiator, so I can just install this duct system (DIY of course) right up to the kidneys.
Here's what I'm thinking:
DIY the stuff circled in red. Either made of metal, plastic or custom made CF

The turquoise would be where the MAF is positioned.
Now for us 323/328 owners: We can get gains by installing an M54B30 intake manifold, and they are not minimal either, as far as I've heard. Good part: THAT'S STILL POSSIBLE by going with the frontal RAM intake DIY.
