The 16 GB arrived yesterday and has been installed.
I haven't stress tested it yet, but it does seem a bit snappier.
I was surprised that the video RAM allocation only changed from 512 to 768 MB when I went from 4 to 16 GB. The mini uses Intel's Integrated Graphics, which is allocated a chuck of RAM from the system RAM.
The Aggregate Device glitch, mentioned in these comments, is still occurring. I haven't figured out a reliable way to make it occur, nor a reliable way to fix it when it does - so I just "fix it" "fix it" "fix it" until the Clock Source stays on HDMI instead of Built-in Output. I ordered a new cable, to go directly from the mini to the receiver, to see if eliminating HDMI from the equation will take care of the problem. Shipment tracking shows that it's on the UPS truck for delivery later today.
I finally figured out how to change directly to a sub-channel without using channel up/down or the guide. If I typed "8" or "13" the channel would change to 8-1 or 13-1. I tried typing 8-2 and 13-2, but that didn't work as expected (as + and - are the up/down channel changers). Typing a period, 8.2 or 13.2, was the trick to make it work.