Chris,
a fuel that burns slower does not enrich the mixture. The FI system throws in a pre-determined amount of air/fuel. A higher octane fuel just prevents detonation nothing more. Unadjusted, the amount of air and fuel remain the same, lean. The bike left the factory with settings lean enough to pass emissions. Adding a less restrictive exhaust just allowed for more air to pass through but not more fuel unless intentionally increased which in this case it was not. Not every bike computer can handle the drop in back pressure and "self adjust" adding more fuel for the performance pipe.
To richen up the mix, you have to adjust the FI system to allow for more fuel to air that's going into the motor.
A slower burning fuel in an insufficient quantity will still result in a lean mixture. There's nothing magical about high octane fuel. You still have to add more of it to the engine to call it rich. I would bet that if Joe simply switched to high octane a new set of plugs will come out looking pretty much the same.
If not, I will buy you a beer...
Ken