after the mods i have done to my z, my mileage went from 44 mpg to 38. a significant loss in economy but its still damn good. well worth it for the power i have gained.
I have run 87/89 octane in my '03 for over 30k miles...... I think you are over thinking the octane ratings. Its just an anti knock rating.... it doesn't give you more HP, make you run faster or jump higher....Its your money, you do what you like.
On a different note, I do usually always fill up at a "top tier fuel certified" station.
my 2012 z1000 manual says "use 95+" octane gas.Its nothing to do with horsepower. I have just noticed that when I run the 87 octane my motor on my Z is noisy as hell. With 91 octane its quieter, so now with 89 it seems to be okay. Its the gas here. I have noticed in my car which runs 91 octane that I get pinging sometimes with the 91 here. I have even noticed when we are traveling I get better gas mileage with other cities 91 octane. I think its just bad gas here in my town. The 87 and 89 are the same price anyways due to the ethanol content here.