I am afraid this is more about the M135i, but I guess it applies to all the F20 1 series, because every model seems to be running relatively small boost.
Through online sources (google search really) that the maximum boost for M135i is 10 psi (~0.68 bar)
. I know it's complete different business but the A45 AMG runs at 26 psi (~1.8 bar)
. Meanwhile a "stage 1" modification for an Impreza is, by memory, around 18 psi (~1.2 bar).
Many people say that the twin scroll turbo isn't too good for boost modification, though explanation was never offered. I wonder how much more boost can be stuffed and how much more power power may be gained through playing with the turbo. Your say?