daihard
Ready to race!
Cars not tuned for higher octane fuel will see zero benefit from running it - performance and mileage will be the same (or possibly slightly worse, as higher octane fuel is tougher to ignite).
Cars tuned for higher octane fuel, like the GTI, will see both performance and quite possibly mileage increases, as they can run a leaner mixture without the danger of misfiring. A leaner mixture means less fuel in the fuel/air mix (or more air). I'm pretty sure you can see why it's quite possible to end up with better mileage! This is also why some with tunes have seen highway mileage creep up a bit - the APR/GIAC/Etc. tunes assume you're ALWAYS using 91 or 93, so they push the envelope a little further towards leaner mixtures (generally by pumping more air in from the turbo). They also monkey around with the timing to take advantage of the mixture, as well as a number of other variables. Not quite as safe, but you get way more power out of it and slightly better economy to boot.
As for the "why", basically, a leaner mixture is more prone to preignition/detonation, but it's also a more potent mixture. Running rich is safer, but causes more pollution (soot, mainly) and produces less power.
I understand what you're saying. I certainly appreciate all that information as technical details like that will always educate me.
Now do the hardware components play a big part there? For instance, you talk about air/fuel mixture, ignition timing, etc. Are they all controlled by software? If so, it should be able to adjust to the lower potential of regular gas without causing much (if at all) extra wear and tear to the engine, shouldn't it?
Now this may be off, but if using regular gas on the GTI were to be as much of a setback as you and Tailwagger explain (i.e. shorter engine life, more wear and tear, etc), would it really be wise for VW to simply state that premium gas is "recommended for maximum performance"? I would word it differently - such as "premium gas is strongly recommended for maximum performance, longer engine life and better fuel economy."