So, since the original refresh, I've also added a new upstream oxygen sensor (NTK OEM style) and that seems to have really smoothed out cold start and idle behavior. I was under the impression that O2 sensors either work or they don't. But I'm starting to believe that when they get "tired", the car may not be throwing codes constantly, but replacement can make a significant difference. I have an O2 sim, so the downstream sensor doesn't matter as long as the heater circuit in it still works...