If you're having notible gains in MPG with the premium, I'd bet you have too much advance on your timing. I had many problems with lack of power (Even with S/C) until I discovered my Timing was ~5 Deg. Too Advanced... No Wonder... Gained about 40 Miles per tank or about 5 MPG. (Edited to add, I get about 14.6 MPG now compared to about 9 MPG! If you really want to change your performance I recommend Port/Gasket matching and cleaning up the castings in the manifold. I did this on a 87 Buick Le Sabre and gained about 10 MPG, too bad the damn thing fell apart. ) Nice to see the Trip meter reach over 200 Miles on a tank.

I would bet your last 10K are on different Gas I.e. Summer Blend or different gas station (Brand). Check the Timing and your air filter, then go from there.

Comparing the X to others is Apples and Oranges. Both edible, but different.

Good luck.