Okay, sadly I spoke too soon. When I tried it last night I wasn't in my studio, and could only test with the built-in audio. On my Audient ID-22 (USB) this morning, playback is about the same disaster it was before.
Switching to built-in audio it's much better, but I still get occasional clicks. Also, watching the Performance Meter, with nothing playing back at all—i.e., the transport stopped—I'm still seeing random CPU spikes on the last thread. Switching from 8 threads to 10, the spikes move to the 10th (i.e., last) thread, but just keep going. Re-launching Logic the spikes go away, but only until I playback once. From that point on, the spikes are continuous, whether I'm playing audio or not.
This is ridiculous. I don't have time to do a thorough plugin debug test right now, but I'll try to do that either tonight or tomorrow morning. I did get notified by Softube that all their plugins are now "officially" compatible, but updating made no difference.