Think you should look into the science of understanding what a knock sensor does. GM has magically discovered a way to get a turbo engine to not require premium gas as most if not all have in the past. Knowing that they are relying on a sensor(that could fail) to detune/retard timing to do this you would realize how wrong you are. If the computer is detuning the motor that means it makes less power. with 87 you can feel this every single time you accelerate from 1500-2500rpm with a pulsing, surging, wobley feeling of the power(can also see the data log KR with the right equipment).
Since you dismiss seat of the pants feel, are MPG differences scientific enough for ya? With 87 octane I can constantly get 38mpg on a 240mile trip(all highway). On the same drive with 91octane premium I get 42mpg consistently (on a few of these trips have gotten even higher). On that trip its 4mpg average better, overall I am seeing 2.5-3MPG better with premium. Strangely though the more city driving I do the less I see a difference in MPG.