The regular HJS cat is the best of performance vs effectiveness, the Milltek cat still passed the sniffer with E50 but not O2 OBD2 readiness but that is fine in Sweden as long as there is no CEL active.
I run it with a 3" Scorpion CB and it is pretty dang loud when you get on it.
The Euro 6 EC...
Yea I dont think there are 3 foundries in Taiwan making Garrett style bearing housings. I know Zage is a huge manufacturer that others order parts from so Kinugawa might just assemble now since the co-founder split off and started Mamba.
About 20hp with broader powerband and less heat/stress on same boost, with JB4 clutchsaver map 6 I see no issue but on aggressive tunes overboost and compressor surge is common with free flowing intake and exhaust.
Pretty sure the 675 is a Kinugawa GTX3071R CHRA with reverse wheels and the 73mm exducer quoted by Tomi is the OD since they all have extended tip.
Kinugawa Ball Bearing CHRA Core Gen II GTX3071R - Cartel Australia (cartel-aus.com)
You can try with the IS38 but GTI fourth is a bit taller I think, how did you set timing in your revisions?
If you set boost with moderate timing curve and added timing till knock after then just backed off to fininsh the tune you could try that earlier revision with 94, atleast if it was...
Nice run and fast shift but try from 4th the fastest manuals with R/Cupra gearing starts from fourth, a Golf R with Vortex XL has it with a 6.44 in 100-200 I think. Turbo must have low enough boost threshold though, tried to get a guy to try it in his Cupra but he was slower with a TTE535.