Re: Electrical power required by engine?
Disconnecting the alternator is a trick (illegally) used in some race series. Gives a noticeable edge if the two cars are closely matched. Not sure about how much extra you would gain.
An old, bog standard (points) ignition system I was using for a project once drew about 4amps (average) with the engine running.
As for an Alfa, you'll have to run the ECU/injection system/fuel pump as well, and I thing the current requirements would be higher. I believe that the ABS system also pulls a lot of current when operating (although this isn't really an issue for full throttle). I don't know how any airbag or alarm systems would react to disconnection of the alternator (the voltage supplied would be lower on battery).
Best guess... Look for the ECU, fuel pump, etc fuses, add up the total rating of these. The normal current draw is probably between half and 3/4 of the total.