Tesco Super Unleaded 99RON (UK)

I've used Optimax fuel from day one in my WRX. Following a tip I tried a tankful of Tesco 99 and am most impressed. Crisper acceleration and more bottom-end torque...it's really noticeable... what a difference 1 octane makes. I would never use supermarket fuel under normal circumstances but am almost persuaded to change. Almost, because I don't know if Tesco fuel has the same level of cleaning agents etc as Optimax and can't find any info. Can anyone clarify this point? Thanks.

-- Clive Norris Selectron (UK) Ltd Musical Equipment Distribution