I've run both the Sunoco GT100 and GT+ both as mixers to other fuels and nearly straight with no problems.
The stock ECU will take advantage of the extra octane if given a few miles to dial in better timing. Most improvement seems to be in the midrange rpms and you will feel the difference in only a few miles of driving. If your running a Utec or some other system that hard codes timing and fuel changes, then you will need to tweek your maps to get max benefit out of the higher octane fuels other than the safety factor of more octane to work with.
My recent best time on the 13G was run on the GT+ at 66% of the fuel in the tank.
The GT100 is good up to about 15 psi max boost here at 5800 ft altitude, but its on the edge, so I need the GT+ if I'm at the drags and running high boost.
Around town about 30% of the GT100 seems to work just fine mixed with 91 premium if your running sane boost levels. I go to about 50% or so GT100 when temps get up in the 90's.
The Sunoco GT100 is 2.7 % oxygenated so in spite of the aromatics it contains you shouldn't have much change in AFR. Its an approved street legal blend, where the GT+ is not technically sold for street use.
To my knowledge the GT+ 104 octane is the highest octane unleaded Sunoco makes, The Sunoco Standard 110 has lead in it.
The highest octane unleaded I know of is made by VP and is 110 octane, its a special blend fuel intended for water craft racing as I recall.