Shorai now recommend a 21A for the 1200 GS Adventure. I tried the 14A middle of last year and it was definitely hopeless for the big bikes. I've just installed a 21A to test for the local dealer and it's a huge improvement over the 14A.
Shorai do state they don't like starting in the cold and suggest you turn the ignition on for a few minutes to start drawing on the battery. Problem on BMWs is that the lights don't come until the bike starts, so the draw is that much less if you just turn the ignition on.
Otherwise, my OEM battery was about 2.5 years old and would sometimes be quite sluggish in turning the bike over. I read plenty about the batteries suddenly failing to start the bike without warning and didn't really want to find that out the hard way