91 octane is terrible, 93 is slightly better.
You will not reach MBT on pump fuels, period. Without the ability to substantially increase boost, the majority of your gains with this platform comes from adding ignition advance.
Direct comparison between 91 octane and racing unleaded fuel.
Racing gas concentrate added to 93
MS103 vs Racing Gas Concentrate
Butt Dyno's are not the answer, real dyno's are but track testing is cheaper.
The Tuning School test are important because the car has a supercharger, all they can tune is the air fuel ratio and ignition timing, which applies DIRECTLY to the Focus ST with the stock turbo, since there isn't much more to be had out of it.
Now you understand why people run E30/E40 tunes like I do. I will have a race gas tune here soon because I want to see the difference 1) and 2) it takes five gallons of gas+ethanol to make E40 while I can run 2-3 gallons of race gas for a little less weight.
It works out to about $30 more; $43 for five gallons of Trick 101 octane unleaded from the pump + E85 = $13.26
Race gas typically only requires a 10-20% change in air fuel ratio vs 30% for E85.