All I can say is the old cliche- "you wanna play, you gotta pay"!!! Seeing as how detonation is the #1 killer of turbo motors, the higher the octane, the better... If not, leave it alone and don't take chances.... Even if you turn the boost down and turn the FCD off (easy to hook up a toggle switch) for pump gas and only turn it up/on with better gas... My Z28 is filled with VP C16 constantly and I pay $8 per gallon at the track!!!! It is probably over kill, but since I never know when I might want to hit the "button" playing around with someone, I am not going to risk a $9000+ motor because I didn't have the proper fuel to protect it...
Not saying you need to run around with race fuel all the time, but since you can only get 91, it might not hurt to at least mix it if you want to up the boost...
Joe