When I ran the Skyline GTR many years back the engineer who did the mapping and tuning for them told me the following.

Very modern ECU can adjust on the fly and so putting in high and normal octane is acceptable and it should adapt on the run. This is to assist them in the most efficient lean burn. I gather this may not apply to VW's ?

Modern ECU will need to be run continuously on high octane to really adjust and work well with it. In some cases a "trip" or reset can make them re-learn and optimise. Cheap fuel in the mix will cause them to drop back again to avoid pre-detonation. Many will then stay in the less aggressive pattern so you will not get any benefit of a tank of the good stuff.

Most of the ECU out there older than 4 years will need manual adjustment with the exception of the early eco and turbo models which tended to have a bigger brain to allow adjustment due to their needs.

For the Skyline he said always use high octane and fixed the adjustment. I used to carry a small pot of octane booster for when I got caught short of a decent garage. That was an aggressive engine and you did feel it if you put in the cheap stuff.

His comment on the petrol was that there are three refineries in the UK that serve pretty much every brand of petrol station including the supermarkets. They blend to order. Branded fuels get more additives to improve burn and cleaning but the basics are the same. He said Shell 98 will burn better than Sainsburys 99 in the engines he used to tune.

He used to do passenger runs with a laptop attached to the ECU in customer cars to adjust and map the tuning. He told them to stick to the same brand of fuel or switch to a lower map (three maps in the ECU with a toggle switch) where not available. He would do a couple of runs on Shell 98 and tune it to the edge then run on lower grade and you could see the ECU pull back in real time due to pinking. This made certain the customer understood the difference and protected the investment in the engine. It was the only time I have seen the difference in real time and it could be felt through the drive.

Even on the last Mercedes and Audi I have now I cannot really tell the difference. Given they have more power than you can sensibly use on public roads it would need to be an odd situation for me to feel the difference. Neither of these gines or my driving style will let me see a difference in mpg so I don't try and clock that.

I prefer to put in good quality normal petrol given the price hike they add to high octane. Unless I know I will be driving it hard for some reason.

Things I have seen instantly benefit it far more.
Tyre pressures, this is the instant mpg killer when they are low.
Eco Mode/Performance mode. The mode buttons on modern cars adjust the gear change up points and engine map in most cases. These can have a rapid 2-4 mpg difference.

Everyone loves a Morgan. Even me, unless it's broken again.