I think AndyF has been though this before, Ultimate (and I think any SUL) has a higher MON value than Optimax, if we were Yanks using R+M/2 then Ultimate would have a higher pump octane rating than Optimax. Remember, Optimax, for all it's fabled '98.6+ RON', is only graded as NUL, not SUL.
I'm happy using anything, although the price difference between Optimax and Ultimate/Esso SUL usually means I end up on a Shell forecourt. Yes, I am a stingy *******
I do recall back in the day when Optimax first came out that I immediately noticed better pickup and power with it, but nowadays I can't say I feel any benefit. The cynical part of me says that the first 6 months or so of production at Stanlow were ever so slightly tweaked to garner a reputation that seems to hold to this day!