If you really want to determine if it's the filter, then you should change only the filter, leave the PP (and don't forget to check your oil level before and after doing so).
If you change the oil AND the filter, then you cannot draw any real conclusions should you actually see your mileage improve to its previous range.
Regardless of what you decide to do, I agree with the others that you should at least run a couple of tanks through it.
FWIW, with my Miatas, I only got more than 20 MPG average for a tank on one occasion and that was when I went on a long road trip and made a concerted effort to keep my RPM's as low as possible and drive like my grandma.
Instead of 20 MPG, I got 33 MPG. I had been on long trips over similar roads before but only got my usual 20 MPG. The only difference that trip was that I intentionally changed my driving style to see how much difference it would make.
...and I also had a 275 lb passenger in my car for half the 6 hour trip when I got 33 MPG.
Pay close attention to your driving. That could very well be the real source of the change.