I'm getting about the same MPG (260-280MPG) in the petrol fr
I used tesco 99 octane petrol in my fr 20vt once and the MPG was exactly the same (I even did an ecu reset too)
also the responsiveness was no different, well if it was better it was not noticeable to me
so it's back to the supermarket stuff...................no probs here
You have to use at least 4 or 5 tank fulls for the effect to take place. The engine needs to clear out all the old stuff, and then the ECU takes time to adapt to the higher octane stuff.