I dont think a dirty fuel filter will cause bad fuel mileage, if anything, cause you to run lean (even though the entire system is regulated and computer controlled) Decrease in proformance, yah.. But, you did an engine swap so you probably know more than me.
Everything he said, plus if you buy crappy gas, you will find yourself pushing the accelerator down further to get the same power effect that you use to get with better fuel. Try other gas stations, run the tank to almost try and then put 15$ (like, 5 galons) in the tank from one station, drive around for a while and see how that proformance is, then try another station.
It really depends on what distributer the particular station buys it's gas from, and you never can tell. Sometime a station will have crap gas, sometimes it has great and powerful gas. And no, I'm not getting a placebo effect.
Pour a dose of fuel injector cleaner with about 1/4 of a tank left (so the concentration is higher) and on the next tank, fill up the tank with fuel (don't buy X dollars, fill the tank) This may help if you have not done this in the last 30,000 miles.