Making a comparison run is impossible, only a dyno run (engine only) in a controlled environment would make sense.
The ECU needs to adapt first, this could take some time and the fuel tank would need to be completely empty. Also air temperature/pressure and humidity would need to be the same too, impossible.
Regarding the various fuel tests in the press (please take in consideration how much power my car has and that it is turbo charged):
http://www.autobild.de/artikel/shell-v-power-im-vergleichstest-41875.html
http://www.adac.de/infotestrat/tanken-kraftstoffe-und-antrieb/benzin-und-diesel/premiumkraftstoffe/?tabid=tab2 (ADAC...not always competent...no turbo charged cars, maybe because they knew that there would be an increase in power and the article wasn't supposed to unveil that and they wrote Boxster wrong).
It is also interesting that most publications were actually using the ADAC test to come to the same conclusion.
I also agree that 5% are a joke on a car with 150 or 200 hp but triple the result and it gets more interesting.
Tuners, who do a lot of engine dyno testing, usually recommend higher octane fuel.
I also talked to various "sources", like I said before and they confirmed the (somehow limited) benefit.
Just try to imagine this scenario: 35°C air temperature, high air pressure, 90% humidity. Assuming that your GT2 has 620 hp, it would probably produce "only" 590 or even slightly less under these circumstances. Using VPower, you probably still would have 620 hp, if you get my point.
Try to think of it as a larger intercooler on a turbo charged car: It may not increase power under normal circumstances but it keeps the power from dropping under extreme driving conditions.
I'm not engineer, so I can't explain it too well, sorry.
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S, BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4