Simple answer that all the forums seem to agree on, and that my garage also approves is that you will get better airflow to the manifold, however unless you also adjust your ECU, and potentially replace your Mass Airflow Sensor (depending on exactly what you are changing in the airflow path) then you may have problems.
I had the full Prodrive Performance Pack added to one of my Imprezas, and the derestricting the inlet was only required for one of the performance boosts for the turbo. Without the PPP, this derestriction would not have given me any significant difference.
(I have had various modified Subarus over the years, so I chat to my local garage a lot :-)
