I've got a 3s and a 5s 4000mah 20c and the work very well. They are much like my Neu pack in that the balancer almost never blinks when the are charging. I don't know about charging them at 2c though. It said that it was ok on Amain when I ordered them but in the warning sticker on the battery it said not to exceed 1c.
Here's the sticker:
