Well just an update. Finally got my ST back. They replaced the entire transmission. It took about 2 weeks to get the part in. I have to say I'm a little impressed that they went straight to full replacement and skipped piecemeal repair. I'm not entirely sure what the damage was, getting a straight answer was nearly impossible, but I have to say so far I'm pleased. Not pleased that a vehicle with less than 30k mi on it required a transmission replacement, but at least they did the right thing and gave me a new one. I'd suggest anyone with issues to be very stubborn and demand replacement. YMMV but my advisor helped get it done. Only put about 50mi on it so far, taking it easy, driving it like a break-in period, but so far so good (knock on wood). I haven't even put it in sport mode yet, which always featured some hard downshifts, so I can't comment on whether those are gone yet.
I believe (based on what service advisor told me) the transmission issues were only with a few months of manufacture. However I don't know the time period. It seems like fluid leaking from seals is what precipitates the problems. Anyone who starts seeing hard 6-7 downshifts (there's a bulletin for this) needs to get in for service IMMMEDIATELY. That's how it started for me, then to power loss on acceleration, then pretty much complete destruction of the transmission. Get it early. Check for fluid leaks (it's a very noticeable smell when it gets on something hot). That's all the advice I can offer right now. Hopefully things continue to work out. I'm glad I got the extended service coverage though. Almost didn't.
Meanwhile the 2020 PIU we have at work and beat on relentlessly has had no issues. Go figure.