I've used shell optimax for the last 5/6 tanks. There is a marked improvement in throttle response, you notice the difference more when you switch back to normal unleaded, the enine is not as smooth. I wouldn't say its any more powerful just an improved thottle.
The only reason I use optimax is for the cleaning characteristics, I do a lot of very short journeys.
A couple of years ago I owned a 146ti but had a problem with a jammed fully open throttle one day, the garage that repaired it found the butterfly to be completely coked up, hence why I'm now using optimax.
Previously, 146TI, 147 2L, Brera 2.2, Mito 135MA