all depnds on what you have done to your car. In my M, I have an intake and exhaust, which normally wouldn't matter, but with the chip that I have in there, it has leaned out the mixture as much as possible. If I run anything less than 93, she knocks pretty bad...
93 for a GT seems a waste. My GT runs quite well on Shell 87. If it doesn't knock, more octane is a waste of money. All octane ratings do is define the ability to avoid knock at higher compression ratios. There is no scientific evidence that it adds power.
Well when I had my GTS it was always Chevron Premium. It is THE best. If I can't get that then shell or BP. If not them then Phillips 76. I avoid Citgo, Mobil and any others or generics like the plague.
Oh - Sunoco 94 is a nice fuel too, but damn hard to find most places.
GTS is 11.5:1 compression - 91or higher octane is a must. The car does not run as well on lower octane.
GT - 87-89 octane is ideal. The car is not tuned for high octane and hence will never advance timing far enough to make the premiuj worthwhile.