I think the supermarkets have had bad press due to a couple of scandals (silicone contamination in 2007 knackering MAF's etc.), but on the whole it will have to pass fairly standardized tests.
I use it when I run out of range of Shell (I won't use BP - don't ask), and use Momentum 99 or Sainsbury 97 tat interchangeably and despite having a stage 2 car I couldn't feel a difference with arse dyno. Do I think running on Shell may give you a cleaner engine over say a 10 year period, probably, but then the benefits of this could be wiped out if you did lots of short / cold start journeys.
I think it's easier to perceive (when you are paying for more expensive stuff) that it does feel better, but if any of you can feel 2-4bhp difference at 330bhp+ then you've got a more sensitive backside than I have.