It's gonna blow! Call the dealer! Ford ****** up!
Seriously, it'd be indiscernible, any difference between 91 and 89 could easily be explained by a myriad of other environmental or gas quality factors.
If it isn't pinging, run the lowest octane she'll take, that's where max power is.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto tune, 100 shot on BBR tune, best et: 11.4 at 120.64
2012 GT, Candy Red, Brembo, 3.73, Tech, Comfort, etc. Procal tune, Roush upper, UMI LCAs, Steeda red bracket, GT500s, 28X10X16 Hoosier slicks, best et: 11.91 at 115.23 RIP