I'm using premium unleaded Andy and must admit haven't gone anywhere near sport+ yet, just left it in Comfort and have only done 513 miles since beginning of March.
Millduu, our mpg seems exactly the same, like yourself I normally get bored with cars and at moment can't see that happening, nothing else out there that I prefer unless I win the lottery.
2017 GLC AMG43 premium plus, night pack, leather, intel lights, 360 camera, side steps. = sadly gone
Now GLE450 Premium plus, Selenite, Black/White Nappa leather, Tech plus pack, Driving assistance pack, Tow pack, Off road pack, 22 inch wheels.