Sorry for the non ford question!
Ive asked the question on a subaru forum, but My general thinking is that they dont have the vast members and knowledge that we have on here, so here it goes; The car in question is a 2002 impreza WRX,(ecu controlled boost)
After putting the cat back on my car it is running less boost, understandably there will be a small restriction now in the exhaust, but IMO this should not decrease the boost. As i see it the ECU will want to achieve target bost of 19 psi (as before) And so should bled off enough air unril this boost is reached, regardless of backpressure. As a test i bypassed the factory boost controller and used a bleed valve, and yes the car can pysicaly run 19 psi in this manner. So my next train of thought was that i may need to "help" the factory bleed valve as it prehaps cant flow enough now, so i installed a restricor from the compressor housing, no change.
This leads me to the conclusion that somehow the ecu thinks it is reaching target boost, so i can only assume that the ecu does a calculation through the air mass meter? Or is there a seperate map sensor for this? thanks in advance.