That statement is true with older Mustangs, but the 3.7L Cyclone motor in the 2011+ Mustangs has adaptive learning capabilities. With multiple knock sensors, the ECU can detect the use of higher octane fuel and change TiVCT settings to benefit from it. Independent tests have shown up to 6hp improvements from just the use of 93 octane fuels. While 6hp is not very noticeable on the SOP gauge, it is a benefit nonetheless.
Personally I have noticed very little MPG improvement from the use of higher octane fuel and in my opinion the cost outweighs the benefit.
Here is a link to an article mentioning the benefit of using 93 octane fuel.