I just found out something interesting. Excuse me if it's already old knowledge:
Recently fitted a boost gauge to my 16VT, referenced to the inlet manifold, and was wasn't impressed with the erratic boost. 1.2bar dropping to 0.8ish (as it's supposed to), But then boost dropped off significantly above 5000rpm. As though the turbo wasn't man enough for the job.
Fitted a manual boost controller (mbc,prv - whatever you wanna call it) and that made worse. Boost was all over the place, depending on what gear I was in, how quickly the throttle was opened, etc. And boost still dropping to 0.6bar above 5000rpm.
So I removed the compressor outlet pipe from the mbc and swapped it for the pipe from the dump-valve. So now the mbc reads the inlet manifold pressure instead of the compressor outlet pressure.
And Presto. Steady 0.9bar all the way from 3000rpm-redline - in every gear - all the time.