A misfire would be unrelated to the fuel octane. Sometimes knock might feel like a momentary misfire though, since the ECU may rapidly retard the ignition timing enough that you can feel the power loss, but if 91 octane hasn't solved the problem, I doubt that's the issue.
If your scanner can read misfire events in real time, you could use it to confirm the misfire and figure out which cylinder(s) it's coming from. If not, you might be able to force a code to appear if you can drive in such a way to get enough misfires to occur in a short enough period of time.
Otherwise, I'd start with the plugs and coils, and also clean the MAF sensor, since it's usually what controls fuelling at high engine loads. If those don't help, check the fuel pressure or replace the fuel filter. It might be good to do a compression test while you have the spark plugs removed as well.
