Wow... The first time I have to comment on anything on this site because this track is sooo good that I really want to contribute to it getting even better.
Three little things prevent me from giving a 5 star rating:
1) Normally I like when there are houses next to the track of Eifel flat as it makes the scenery more interesting. On your track they are a little disturbing as they are soo close to the track and are not there in real life. especially the two houses in "Hugenholzbocht" and the church after "Arie Luyendijkbocht" do not fit there. Maybe you can move the track around a bit to get rid of them. They should vanish if they are too close to the road.
2) The "Tarzanbocht" should have a sandtrap on the outside. In real life it looks much wider and "spacier" for that reason. I think you can change the curve from a guardrail-curve to a sandrap-curve with the new tools.
3) The whole track feels a little too narrow for my liking... especially on the "Kampf der Zwerge" video you can see that it is quite wide in real life... I am sure that this is Polyphony´s fault as their scaling seems a little off... if you set the track to 10m (like I think you did) it is more like 9m of space between the curbs in GT6. I made a picture to show what I mean on the straight right after the "Tarzan":
https://www.gran-turismo.com/gb/gt6/user/#!/friend/Cooya_1/photo/
The BMW is 4.2m long... if you put 2 of them on the track there is almost no space left what makes me think the track is 9m wide instead of 10... You can read about this issue somewhere here in this forum, too ;)
Everything else feels just perfect. I just love how you nailed that rollercoaster feeling all around the track!
I.e. Audi corner is bouncy looking IRL and I left it bit on this too.