Well on the 14th of this month (January) I put in 4.17 gallons of Esso Supreme+ and despite my association with B.P. fuel, I actually found my car went really well on it. Low down pulls in 3rd gear were actually really nice, and the car felt so smooth under acceleration.
I've done a few 2nd and 3rd pulls, and a bit of back road bashing, and the ST has managed to average 26.6 mpg, which again I'm pretty impressed with.

I think next on my list is lambda sensor, cat, and maybe something else