ok let me explain something about the new mustang and the new engine managment.
it has a VERY sensative and acurate knock sensor..
it would be very hard for you to hurt the motor naturaly aspirated even with a 93 octane tune and sticking in 87 octane and going out and hammering it I doubt you would even hear any detonation...
the knock sensors will pick up the inaudible detonation long before you hear it and then pull the timeing out until it goes away..
is it a good idea ? no.
but will you seriously injure your car.
only if whoever wrote the 93 octane tune limited how much the knock sensors can pull out or stoped them from pulling any out at all.