I agree that 12.5V drops are nothing, really. Batteries are 12.8, so realistically, you are only dropping .3V before you kill it. I know there's more to it than that, but once you pull over 300 amps, you're going to go to battery power, which is normally 12.8V.
My Ohio 300 amp alternator puts out pretty much nothing at idle, and I was told by a couple other manufacturers that in order to make 300 amps, there has to be some give, and that's charge at idle. As soon as I touch the gas, my amperage and voltage comes up to normal, but at idle, I'm also running off of straight battery power, pretty much. In contrast, a couple friends have 200 amp alternators, and theirs put out decent power at idle, and it's the same alternator casing, just mine is 300 amps, their's is 200 amps.