My 15 GT has 1200 miles and has only seen 93 octane. When I picked her up at the dealer, the salesmen gassed her up before I left and he said to only use 93 octane (he has a GT as well). He even said that you won't notice the performance difference, the engine will run more smoothly.
Right Now I'm trying Shell 93 V-nitro to see how the car runs. I just did my first tank and will do 2 more. Then I'll Try BP for 3 tanks and then Exon / Mobil.
Right now the only thing I know for sure is the weather effects the performance more than anything. Hot over 90 humid days, sucks the life out of the car.
Its been about 3 tanks of the Shell 93 nitro, and I can say clearly that I'm getting about 2 miles more to the gallon. Averaging about 21.5 instead of 19 on my daily drive to work 32 miles round trip speeds of 75 average hitting hard when I like. The car runs like its on fire (As long as its cooler than 85 degrees outside)...Just my experience.
Meant to get back to this before now. I am probably off base here, or mistaken, but of course Ford recommends 87 octane. They don't want their soon to be discontinued base rental fleet cars producing more HP than the Eco boost. I would love to run some tests on the dyno with mine, but given what the computers on these cars is designed to do, variable valve timing etc. I think it is a safe bet I am producing more than 300 HP. Just my opinion. BTW, had a chance to run 87 a few weeks ago. Car felt more sluggish, had a rougher idle and just flat didn't perform as well. 92 octane it is.