It's a wash in my car with the cost per mile being aroun the same. The car knows what you're feeding it and the extra 1.5-2mpg you gain offsets the cost of the fuel. I also pick up about 2 tenths and choose the better fuel for consistency. At least until the hottest day I've ever run happened. 
The fuel computer knows it too when I would fill up a second tank of 93 after 87 and back again. Very different mileage numbers and according to Vehicalc better results. YMMV, literally 
Golfer old friend the car doesn't (computer) know what you put into it for fuel. It however does know how much air and fuel it is set for (PCM)
The PCM in most modern cars can be recalibrated. For MOPARS it's a simple matter of pulling Fuse #2 for 30 seconds. Then starting and
driving like ya stold it

Believe it or not the PCM has a sort of memory on how you drive. Find out the procedure for your beautiful Mustang.
The only way you'll tell the quality is by how the car runs. If it says 87 from Ford you should have no problems with predetination, engine knock etc.
If you get better gas mileage it will be minimal. Same with performance....if any. One thing I do know is any automoblile tuned correctly will run
better on low humidity days.....usually those days are also cooler. If however you have two days 80 degrees, the one with the lowest dewpoint
will make your car run better, less water taken into the intake with the air.