I have had excellent results running 100 unleaded in my tuned Turbo both with and without cats. Sam @ By Design whipped me up a custom 1.0 bar tune that's optimized for the 100 octane, Kline exhaust, and D088 ICs. It's presently $8.97/gal here in NorCal, which is less than $3 more than 91 octane, so for the amount of driving I do it's a good solution.

back in the late 90s I had a supercharged Mustang 5.0. It made good power back in the day, but was always held back by crappy 91 octane. I had a roommate that worked at a local muni airport and he regularly brought me 5 gallons of 100LL which I mixed in with 91 pump. I didn't have cats on this particular car, but it did mess with the O2s, primarily in that it would cause a surging idle. I don't recall having to change out the sensors, but switching back to straight 91 and the surging would go away.

Running 100LL in a 996 may yield good short term results, but I've gotta think it will foul the 4 O2s fairly quickly...