Well the HKS GT-RS's that came off my car had NO restrictors in the core BUT had really stupidly small oil holes in the banjo bolt.
The off the shelf Garrett -5's that replaced them had the restrictors in the core hence no need for the HKS banjo's. Stock banjo's went back in.
600 cc @ 90% for 276 kw at the treads... Something wrong there. Rebuilding them x 2, something wrong there. SS's laggy? Wow.
Find someone to fix the setup you have, there is something wrong.
Jez, I had the same thing with my car mate. Brett ended up fixing it. Long story short we blew compressed air through the bleed screw and it was all good.... and has been since (that was almost 12 months ago).