no Sheetz in my area, but Casey's has E15 ( they call it "Unleaded 88")
used it quite a bit in my hybrid, noticed no appreciable MPG difference between E15, and the Normal E10 that every other pump in the dispenses.
the only time i noticed a difference from the Fuel, was when Casey's Sold E0 91 octane. with that I got about 5more MPG's than the E10, or E15...
and it wasn't the octane, I tried a Tank of E10 93, no different from any other E10.
honestly, i also don't notice much of a difference from E10, when i Run a tank of ~75% E10, and ~25% E85. ( mainly as a Fuel system Cleaner once or twice a year, even though I'm using Top Tier Fuel)