Another well-known tuner here in the the US reported this on a Perrin & stock header thread:
Notice how he says the EL headers "lost" power @ higher rpm's
Originally Posted by AZScoobie
I did one on an FP green STI. Turbo came on 600 to 800 RPM faster and as a result made 40 ft lbs of trq and 25whp under 4000 rpm. From 4500 to 6000 rpm power was exactly the same and from 6000 + it lost 8whp.
I've also heard the same comment from another very well-known and experienced tuner. The Perrin EL headers work great in turbos smaller than yours. The engine is an air pump. restrict the outlet and you can't take in more air than you can pump out. The turbine is pumping air into the engine once fully spooled (higher rpm's) but the engine can't use all of it because it can't pump it out. In this case, you will still see your boost reach very high PSI's but it doesn't mean the air is making it into the cylinders.
That's my best guess. Alot of good suggestions from others here. Now, there could be other reasons. So you'll have to carefully analyze where the system is choking.
Good thread.. we're all learning !