depends on what you would like to spend.
filter, de/sports cat, free flow pipe is low end but as it is N/A the returns are not going to be huge but with a map you will get the 2.0 lump running as it should not restricted to manufacturing / Euro regs.
You could look at a full lightening and blueprinting of the engine - lumpy cams, ported + flowed heads (possibly bigger valves dont know??) etc that will get you more but will start to hike the cost.
I have been contemplating the idea of a 2.5 lump replacement without the turbo setup.
power to weight ratio is a good low cost approach - remove or replace heavy items
Buuuut the chassis is setup up for comfort not speed so gains can be had by stiffening it up (shocks ARB etc), replacing standard discs + pads (possibly braided hoses) to improve the braking capability (they are not bad as standard and give better feel than my STI but do fade under enthusiastic driving)
I have just ordered the ODB and app so will have a crack at the 0-60 times as the standard auto box change program is not great - I checked with Subaru, there are no modification (PAK) files to change / improve it. So that 0-60 of 11.5 may be in standard auto mode. Ill do a run in Sport auto and also me manually operating the box