I've done my own 'highly scientific' tests on super fuels. Up until I bought my mkII as the daily driver I've been running my 911 daily for 2 years at 10k miles p/a. Its designed to run on 95 Ron, so I've started merrily filling up on any brand 95 Ron, usually Sainsburys or BP for the Nectar points. Whatever driving style I adopted it would return 300 to a full tank. I then started filling up on 97 Ron, basically because the price difference at one point was marginal. The first time I did this it recorded 320 on a full tank, so there after I alternated between the two octanes and like clockwork it would generally run an extra 20 miles on the stronger stuff. Performance wise, I had dyno'd 3 times in 2 years, twice on 95 and once on 97, on all occasions it knocked out 241 bhp +/- 1 bhp. So from my own experience I would say you are quite likely to increase your miles per tank, but BHP, doubtful. Maybe on modern stuff the ECU can adjust itself for the stronger stuff, I know on the Motronic my 911 uses there is a little adjusting screw within the ECU for fuel quality, but then I'm not going to remove and open it up depending on which fuel I fill.
Anyway, if you want better mpg, buy a different car, the Gti does the same miles as the Porsche does for £40 less per tank.
