The post for the ball mount that comes with the Ascent hitch receiver is 1". The most common ball size is indeed 2" for the majority of things you are capable of towing with the Ascent (Class III) but some travel trailers may still need 2 5/16", even though they are lighter in weight. It depends upon the manufacturer...some prefer to standardize on the larger size across their lines.
Please note that you need to very securely attach the ball to the ball mount and the required wrench sizes are not commonly in the average person's tool set. It's best to use the right sized tool for this so it's fully torqued down. Pliers are not the right tool... (I use a heavy ratchet wrench I have for maintaining my tractor...it has a very long bar and the correct size socket)
I have not, Sigmonster. I thought about it a few years ago, but decided to stick with discrete ball mounts and balls for 2" and 2 5/16" applications with my Grand Cherokee. In particular, I was using a much heavier ball mount for the larger connection. (small horse trailer) The specifications on that quick change system seem to be good, however.