I've never used Tesco's super and I've used Shell a few times but I've not noticed anything different to compared to Sainsburys Super that I use.
From what I remember from my Subaru days that the Shell and so on, its only really shown big differences when using high boost ie 1.5bar and more for huge boost. I remember one P1 owner having a boost of 2.2bar! I think it was around 550bhp and it was definitely no slow car that was for sure!! :lol:
From what I'm returning mpg wise, I think the lowest I've had so far was about 35mpg and the best so far is just under 39mpg.. I'm not driving completely light footed, but I aint flooring it in every gear either

I'm quite impressed with it to be honest. Figure if I could get rid of this lumpy running problem, I should be able to get near to 40mpg

I have manage 390 miles from a tank full, with around a quarter of a tank to go

I do do a lot of miles running from North Devon to Wolverhampton every other weekend, so I'm pretty happy I aint using it up as badly as my Scooby did!! :lol: But then I'd kinda hope not, cos its not nowhere near the power that that thing had!! :lol: