I've found this:
https://www.amazon.com/ACDelco-24266725-Original-Equipment-Transmission/dp/compatibility-chart/B010GJO6UE
which is a separate transmission controller for the TL-80SN 8 speed used in 2014 and 2015 Cadillac CTS-V models. The TL-80SN is the same as the Aisin AA80E except for some internal changes like different heat treat on some parts and extra clutch plates in some clutch packs. No doubt the bellhousing pattern is different as well, I do not know what they did with the electrical connectors.
However, for this TCM to work with the GM engine controllers it has to have a CAN bus communication protocol that matches. I believe that is very likely to be the same as the one used in the MegaSquirt controller also, as it was developed primarily by GM enthusiasts. Maybe lady luck is looking my way this time, hope springs eternal as they say.
Looks like this is my next approach. Perhaps I can find a CTS in a junkyard and harvest the controller and harness. If not, the TCM can be had for around $250 as a new GM part.
Jim
8 Speed, OOOOOHHHHH
Yes, all I need is a controller. The rest is done.
Jim