Update on my situation: I traded a couple of IMs with tuner extraordinaire Penner and he said there's no need to replace the O2 sensor. He told me to add 6% to each of the L1-L3 tables (they control low throttle fueling), unplug the O2 sensor connector, and uncheck the O2 device in TuneECU and it'll run like a top. I followed his instructions and as usual, he was spot-on! No more Check Engine Light, and it definitely idles smoother and has less popping on decel. It was a bit of a learning curve to edit the maps, and to find the darn O2 sensor checkbox, but the ECU re-flash went smoothly and the bike is running great.
The used O2 sensor I found on ebay showed up yesterday but I doubt I'll even install it at this point.
