Thanks for the information thus far.
Our next step is going to be determining the stock camshaft's profile. From there, we'll be able to decide if a new profile will be worth our while.
As for swapping heads, I still think this is the ultimate option. Granted, it might end up costing a bit, but even with stock 2.5L NA SOHC heads, you gain 20-25% in flow!
In regards to the WRX heads, they uses thicker valve guides in their casting. Because of this, they actually flow worse, in stock form, than the NA 2.5L heads. They'd respond well to porting, yes, but you would have to put this extra time and money into them to get the benefits. Otherwise, the DOHC or SOHC 2.5L heads are a great buy I think.
I also need to cc the Legacy Turbo combustion side of the head to determine what will happen to our compression when we swap to different heads. Has anyone cc'd a piston, know the piston deck clearance and the head gasket compressed thickness? And I'm sure it's easy to find somewhere but what's the bore x stroke of this motor?