A few of my buddies ran race gas at the dragstrip with their stock E55's and E63's (both V8's, one is supercharged, similiar to our cars) and they broke records by running high octane fuel. I know some people say the ECU takes a while to adapt to the higher octane and some say it makes no difference, but I'm just giving my feedback from what I have witnessed.
I am very interested in this subject also, and will probably run a few tankfuls of about half 91 octane (damn republik of Kali) & half or a little more 100 octane VP unleaded. This will be on a new not even broken in '09 CTS-V that is waiting at the dealers for me to pick up, so won't be any dyno nor track time / runs, only seat of the pants feel, which we all know is not very accurate. There have been several threads on this subject in the last couple of months that were very interesting reading, but no actual dyno or track data supported.
I would definitely not run any leaded gas, leaded substitutes, or -octane booster-. In one of the threads somebody replied that their car did in fact run better by seat of the pants butt-o-meter. Somebody else stated that the ECM will adjust somewhat to a higher level of octane, thereby allowing the car to gain some benefit from a little more timing. Sorry for the long post & HTH
TIA/R: Gary Wells