OK - I finally got my CM3+! (8GB was all that was in stock at the retailer I used) Installed easily and was able to follow instructions to install the CM3 slice version on the eMMC. I probably could have backed up my settings, libraries, add-ons, etc. but in my case it was easy enough to just start from scratch.
Now for the heatsink: Locally, I found a small, fairly lightweight, copper heat sink (20x20x6 mm). With just that guy, the max temp (gputemp / cputemp) I’ve reached by just playing videos is about 52°C. While playing video AND updating a large library over the LAN, the temp leveled out at 60°C.
From what I read at the pi site: “...the CM3+ has better thermal behaviour under load. It has more thermal mass and can draw heat away from the processor faster than CM3. … longer sustained operation under heavy load before the processor hits 80°C and begins to reduce its clock speed.”