95 octane wont affect your sensor, and as for cat being faulty, its out of the question. if your cat is faulty the code of p0420 will be logged and not p0031 or p0032, which is a/f ratio .
when you went to armstrongs, did they install the pak file(ecu reflash) for the a/f ratio sensor? subaru japan had released few ecu upgrade for these models where a/f ratio sensor gets buggered up too quickly.
go back to them and ask them if the pak file is available for your car. get that installed first before you change the sensor.
it happened to by bp5 as well, where the new sensor faulted only after 10,000km of driving. installed the file and it has been 2 years and hasnt faulted yet.
the reflash will not cost as much as the sensor. should be around an hour of labor plus the price of file that they have to acquire from subaru nz