as i've said before, i do think that their recommendation to "warm" the battery helps, but i'm just not sure i'd want to deal with the time that it takes to do so if i lived in a cold climate. i will say that the battery does act as it should if you go thru the proper steps on those colder winter mornings (haven't had any experience below 40 degrees).
as for the cca, the 21a worked best in my GS but you do give up some of the weight savings in exchange.
2012 Vespa 300 Super
past: Ducatis, Aprilias, Moto Guzzis, a Husky, KTM 1290 SDR, and some BMWs as well