Last summer I took my blue bike out west ('15 ST Limited). I toyed with running different in different tanks of gas. I ran each tank down to fumes before refilling. 92 octane got noticeably better gas mileage then 87 at interstate speeds - that is 26 - 27 miles a gallon vs 32-33 miles a gallon. This seems to confirm what's written above regarding performance and efficiency running lower octane fuel than specified.
Always here to fuel the debate!