Anyone knows what the stock boost pressure for the UK spec turbo is in particular MY00 ?

And for those of you using boost controllers does this significantly improve performance and do you have to use a Blow off Valve too ?

I'm not sure about an MY00 but on the 98 models it's from 0.8bar to around 0.95. I'm pretty sure it's the same for the later models. I run an HKS EVC4 boost controller and it has significantly improved performance combined with exhaust/downpipe and filter combination. The Impreza already has a recirculating dump valve fitted as standard. If it's the whoosh noise you want, fitting an induction kit will make the standard dump valve very audible! I run the boost controller at 1.15bar. Bear in mind that 1.2bar is the safe maximum for a UK Impreza turbo and you shouldn't run into any problems. I would recommend running at 1.15bar to be on the safe side. The Ecu can self compensate for fuelling and ignition timing up to 1.2bar. My boost controller cost around £500 from Torque Developments at 07000 TORQUE (Mark). I'd fit it yourself as it's a relatively easy job!
Hope this helps,
Nito