mild performance upgrades for non-turbo forester?

air intakes like K&N and injen work well, improve throttle responce, sound awesome, and are very nice. I dont know about your 20-30 hp goals, that sounds a little high, but in general the air intake and exhaust would be good places to start. The exhaust sound is nice on subarus, they have a distinctive "burble" in the exhaust- a trademark sound of the boxer engine and unequal length exhaust manifolds.

in general, air intake is the way to go. Be cautious of cold air intakes that are placed low on the car though, as they risk water ingestion if you drive in floods or deep puddles. Ram air intakes are nice.

good luck

Subaru Forester Performance Parts

20 to 30 is not out of reach but not with bolt on parts. Your car is tuned pretty efficiently with what it's got so there is little to be wrung out of it with simple bolt on parts. Full header back, cold air intake and a lightened crank pulley might give you the 20hp but in a couple weeks you will get used to 20hp and it will feel slow. Then you just wonder why you spent $2000. And I don't know why you want to insult the XT drivers calling it a boy racer car but you want to add things to your car that will just make it sound like a boy racer car but still slow. The N/A Forester does what it was designed to do better than just about any other car out there. Should have gone with the XT. Whatever your thoughts are of the looks, at least it performs.

Drop in a K&N Replacement Air Box Filter

Install a lightweight Crank Pulley

Install a Cat Back Exhaust System

Keep your Forester tuned up properly.

Loads of different ways depending on the size of your wallet and what sort of performance gains you want. A common tune is exhaust system, induction kit and ecu remap. Is the engine the same as the Impreza 2.0 sport (thats also non-turbo) if so some of the bits might be more easily located.

Leave it the f*ck alone. Unless she's Sabine Schmitz or Michele Mouton.