The ECU manages the turbo for longevity; successfully, too - you could get 300K out of these turbos. It also happens to control boost so as to reduce stress on the engine (not a bad idea for a car with an aluminum block). Hence your numbers are pretty much in line with specifications. For the best rush, spool it up when you're already going 50mph - you'll spike 17-19psi, sustain 14psi, and in a couple of seconds your speedo will be darn close to triple digits.

Some have tightened the wastegate rod, which allows the turbo to idle at a higher spool rate, reducing initial turbo lag. But it won't make a whit of difference to your 0-60 time as the ECU detects this and compensates. But still. Many have experienced 'quicker low-end response.' Tighten it too much and the ECU compensates by shutting off your engine.

You can bypass the ECU altogether with a $35 device called a manual boost controller. You won't be limited to 6psi on launch anymore, but you'll experience torque steer, patch outs and other heady effects. But you'll also put more stress on the drivetrain, your turbo will run hotter - and will have your turbo out of the car for a rebuild at 150K miles, tops.
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David
1998 S70 T5SE // misc mods (mostly lighting) // red calipers
1992 940 GLE // Hella Micro DE foglights and 170K