Haven't read all the thread... But you should be running 98 octane only in any Subaru running the EJ20T as far as I'm concerned
Without a doubt, best bang for your buck is going to be a re-tune. You'll gain some power & torque, plus you can get the throttle sharpened up. It makes the auto much nicer to drive IMO.
Of course, if you are planning on doing some other mods (Major or minor) I'd do them before the tune.
But since this thread is technically about mods on a budget, I'd probably get some sort of panel filter (HKS

) & remove the air box resonator then the tune

If your pretty happy with the Lib in stock form, then you'll love the results you'll get from the above
