from my understanding if you run a lesser octane than the 100 octane it is mapped for the ecu will pick this up and retard the igintion to prevent detention. obviously going as low as 91 is testing the limits the fuel map toyota prob set on the ecu
running 98octane and getting as close as you can to the factory fuel map you hopefully will make gains in effienency in either power or fuel per km's used. or at least peace of mind
alternatively install a fuel/air meter and at least have a general idea of the mix on the exhaust side of the turbo and when you start winding the boost you can see if your rich or lean.
this is my pillar mount on the gtt

