Depends on turbo.
So although there is no official figure
Imo can take huge amount.
So example you can use a samco big dog leg and RCM induction to replace induction, and turbo running 58mm standard inlet as per Subaru original. can to 450bhp area.
I will call it at that. There is no construction design that limits air intake.
I again highly highly advise to stay away from these Samco lookalike chinese copies. I understand attractive at cheap prices. But no.
They are not the same as Samco.