A 1% FE change is very possible, but you are right to be suspicious without knowing how they measured that. Lots of things could cause a 1, 2, or even 3% change in the real world. Most likely they took some really gunked up engines that also knocked, then used their magic potion on it and looked at the change after a few tanks. Thsi was almost certainly done in a lab, not taking customer data.

Knocking is not due to excessive cylinder temp. Well, it is in a way, relative to the fuel being used. Knocking is the predetonation of the fuel charge- literally the fuel starts to explode before its is sparked...explodes because of temps and pressures that are completely normal, but the fuel itself is too volatile.

Higher octane fuel literally means more octane vs heptane in the fuel- octane having 8 cabon atoms: more chemical bonds, therefore higher ignition temp and later ignition, preventing knocking. A lab tests a fuel mix and compares when it started to knock (under an increasing load) versus pure isooctane and pure heptane fuels.

Now the refinery does not actually add C8H18 to the fuel...the cheaper and most often used way to increase octane is to make fuel "equivalent" to 87, 89, or 93% octane by adding MTBE or ethanol. Pure ethanol has the equivalent knocking of >110% octane gas, so a little bit added to stock fuel brings it up to 89, 93, whatever. Every 10% ethanol added kicks octane up by ~2.5%. Tri-ethyl lead used to do the same thing in "hi-test" gas, for those of us who remember leaded gas.

MTBE/ethanol also has the effect of leaning out the fuel- adding oxygen to meet EPA RFG requirements.

Ethanol is very often also the 'detergent' used in fuel. Alcohol is an excellent solvent and scrubs out the rust and gunk from the fuel system- anyone who has converted an engine has seen their fuel filter clog up very soon after going with straight ethanol. Alcohol also clears out deposits form the injectors and elsewhere, and is mixes with any water in the fuel, preventing gasline freezing or a water slug getting to the engine. A bottle of dry gas or injector cleaner is usually straight methanol- even more powerful solvent than ethanol.

Now, alcohol, MTBE, whatever has lower energy content per unit volume than gasoline. So adding it reduces the energy content in a gallon of fuel in your tank. 10% ethanol reduces MPG in most engines by about 2.5%. If YOUR engine knocks, you're losing more than that anyway, so the higher octance will help improve MPG on balance. But if YOUR 4-stroke piston engine is NOT knocking, the higher octane will not help MPG, other than to ensure your fuel system is clear. Very very few engines have knock problems anymore- a "knock snesor" is standard instrumentation and adjusts spark timing and fuel charge delivery to stop it 99% of the time. An engine that requires premium simply means the engineers painted themsleves into a corner when they designed it, and the easiest way to make sure power performance specs were met was to slap a "premium fuel only" label on it.

So the magic juice is usually Ethanol or MTBE, doing double or triple duty. Pretty magical stuff.

As for lubrication/anti-friction, neither the EPA nor any major manufacturer has noted any change to lifetime, wear, or performance with any change in the "lubricity" of gasoline fuel in modern 4-stroke engines (Ethanol reduces it somewhat, and some folks may add something to counteract that). It is not a parameter that is generally measured exactly because it has so little effect. Any changes that were seen often contradicted each other and/or were so small as to be lost in the statistical noise.

Having said all that, your mileage may vary. If Shell works for you, then go for it. But at least you know what's behind some of the marketing blabber. (For the past few years Shell gets its crude from the North Sea and West Africa- NOT from the Mid-East. Another good point, to some.)