91 is ok to use. I use 91 since that is all I have around here as well for my turbo and it runs fine.

The plugs are orange/brown because you are running really rich. What sort of fuel management do you have to fuel the extra air flow that nitrous provides? I wouldn't go any hotter than one heat range colder than stock. Get one range colder or you risk melting the electrodes/detonation.