My 2004 sti is get really bad gas mileage

[xSubaruSTIx24]

Heres the story i bought my car almost 2 years ago. FROM highline car connection and i know that place isnt a good place to buy it from but i bought it because it HAD 16000 miles for an 04. BUT when i bought the car it was geting 189 on a full tank and now at 29000 miles im geting 155 on a full tank I should be geting 230 to 260. BUT my car is stock!

But i dont know if i bought my car with the upgraded fuel pump and injectors and they could have may have put it back to stock as much as they can and left a few upgrades on it. Just bc someone told me i forgot who but they said thats why i could be geting bad gas mileage or whoever had my car b4 could have tuned my car for more power. Then others say i could have a bad cat in my exhuast and my friend called subie tech line. They said it most likely could be a bad cat but my engine light isnt on? BUT it came on twice in the time i had but went back off the same day and subaru said that the cat sometimes wont make the check engine light come on.

So any ideas what it could be exactly?