The other day I purchased my l/f autometer boost gauge, fitted it and then discovered that the boost pressure was not rising above 10 Psi. AWD Motorsport (Perth)swapped with another gauge but to no avail...checked with Select monitor but all seemed well. The problem seemed to be eminating from a possibly faulty Duty boost solonoid, so tried in vain to source a swap to test...no luck...I called Town & County (Perth) to see if they had any ideas (you'll have to bring it in so we can cripple you with labour charges!!)they were not sure. Then came across a pic here on Scoobynet regarding the sensor plumbing. It showed that the restrictor should be on the turbo outlet nipple in the longer length of hose (35mm from the T centre), it turns out that the restricor on my car was on the actuator feed (the short length of hose). A quick call to Subaru (T&C) confirmed so I repositioned the restrictor and hey presto 14Psi.
I called AWD to tell them (thanks for the help in tracking the problem down Dave (Brown))He was astonished and never seen the setup before.
So I guess if Boost is a problem then the positioning of the restrictor is worth checking!!!
My car now feels totally rejuvinated...Thanks Scoobynet, AWD Motorsport, Town & County Perth.
EwanS

Ewan,
The reason behind this is that the positioning of the restrictor makes the bleed of air via the solenoid more effective.

If the restrictor is placed AFTER the line to the solenoid then you need to bleed off a lot of air in order to reduce the pressure to the wastegate actuator.

If the restrictor is placed BEFORE the solenoid then the pressure in the line that the solenoid is bleeding air from is lower (due to having a pressure drop across the restrictor) and so you only need to bleed off a smaller amount of air to have an effect on the wastegate.

Hope this helps further with your understanding of why it's done what it's done.

Andy