I remember reading articles about alternator kill switches. It seems like it was more popular back in the old days.
I know two or three horsepower isn't much but it's something.
It makes you wonder though,,, is it possible that the lower voltage to the coil might cause a weaker spark and erase the potential gains.
I know that some racers run without any charging system whatsoever. They just charge the battery between rounds.
I think some of them are using lightweight 16 volt batteries.
My Super Shifters buddy uses no alts and 16 volt batterys in his 10 and 8 second cars.