I'll go at this early and let others disagree.

Tire pressures are somewhat of a personal matter, depending on driving style and how much you want to rotate the rear. I've only autocrossed 4 or 5 times on street tires (I assume you're on street tires), but I start the fronts at about 34# and the rears at about 35. I've been autocrossing for over 20 years, and I still consider starting pressures to be no better than informed guesses. I do know that you'll likely have too much understeer if you go by Porsche's recommendation in front.

The Cayman is best balanced front to rear with a full tank, and the gas won't be sloshing from side to side in turns (and it always moves toward the side you don't want it on). However, with a full tank, you'll also be carrying additional weight (about 6# per gallon). Honestly, I don't know what the optimal fuel load is, even for my own driving style. Consequetly, I run with about a half tank, thus shedding about 48# compared to a full tank.

I've only run 93 octane, so I don't know about a possible 91 versus 93 difference. The Cayman is supposedly tuned so 91 will be sufficient to provide full hp. Even if 93 octane gives a couple more hp than 91, I don't think it would make a noticable difference on an autocross course.