Decided for the last couple of weeks to try Tesco Momentum 99 fuel instead of the regular. As they suggest the fuel is better quality, burns more efficiently etc.
I can honestly say I have noticed absolutely no difference whatsoever, the same MPG driving the same journeys the same style, doesn't idle and differently doesn't even feel more responsive.
Absolutely nothing not even a placebo effect of it feeling better.
The 99 is the octane rating. That is it.
A car built for ~95 will see zero improvement in ANY area of the engine.
All octanes have the same amount of fuel cleaner, the same amount of ethanol, the same amount of climate compensating fuel rating (winter/summer petrol).
Unless you are running a heavily tuned or turbocharged engine there is no need to put 99 octane in your car... ever. The octane rating determines the ignition pressure. Higher pressure means more knock, so you need a higher ignition pressure to prevent it.
Any anecdotal evidence of the engine running smoother or better is purely placebo. These changes are from atmospheric differences and the engine turning over from different temperatures. Also from your ECU varying the timing.
Any difference will most likely be down to the brand of fuel and even then I doubt you will actually be perceiving the difference. Maybe in fuel economy, that's it.
The idea of "flushing out the system" is equally an urban legend born of just perceived improvement.
Any variations in power will be because your engine is setup to run on higher octane. Your bog standard ZS 120 does not run on high octane.