(7) This doesn't apply to all performance cars, but in track racing where you really need to save every possible ounce, it's common to take the battery out altogether after starting the car and run it off the alternator.
Co-founder here. Do the cars you're talking about use a distributor? The battery of a car works as a low pass filter for the alternator, and removing it entirely can cause huge spikes in the voltage it puts out. That can be destructive to just about any electronics it would be running. I've personally seen an accidental battery disconnect blow an ECU.
Good to know, thanks, I wasn't aware of that. I'm guessing the guys that I know who do this have either accounted for that or don't care.
The grandparent poster could be talking about magneto or traditional points-based ignitions, which are still common in certain classes of racing cars.
I've never seen anyone do this. We just run lightweight batteries and keep the car on a tender.