That interesting, the Sainsbury Super stuff is 97 Ron I believe so jot quite as bad as bog standard stuff, my mpg compares with what others get on here and I had good results at Abbey so it will do for me...
97RON will be fine, close enough to what it expects. You may find its a bit over 97RON anyway. Technically 99RON is a little better as it can advance timing ever so slightly, but no a lot.
I definately notice it pretty much straight away when I have had to put a full tank of 95RON in. Doesnt take it long to detect knock and wind back the timing. On the flip side, it does take a while to notice the difference going up to 99RON from 95RON as it will take a while to advance the timing as it doesnt know its 99RON, it just keeps going until it detects knock. As thats quite slow, you dont really notice the difference in the "bum dyno" of using 99RON, just in MPG, but if you ever drop back down, you do notice it pretty quickly. Texaco fuel made my car have a fit, really didnt like that, and I only put a tenna in when it was desperate (DTE on ----)