My 2002 996 was designed for 93 octane, but here in southern California 91
is the highest widely available pump grade. I mix Sunoco 100 into the tank
at the ration of 2 gallons 91 to 1 gallon 100 and get 94 in the tank.

Subjectively, the cars seems to have more punch, particularly in the higher
RPM range where the engine makes max horsepower, but I haven't done any
scientific measurements. What the fuel should do is allow the engine to
operate as it was designed, rather than adjust itself to run on lower
octane.

Aral (a unit of BP) and Shell both market 100 octane here in Germany, but
the guys at the Porsche Zentrum Frankfurt say stick with the 98 octane stuff
unless you actually enjoy pissing money away.  They further add that when
you absolutely must refuel and 98 octane is not available then 95 octane
will do and then mix in some 100 at the first opportunity...