I've just completed the swap two days ago, went without problems.
Left the O2 sensor washer that was already there. I figured out if there's a leak, it's a 20 minute job to change it later. I also didn't put anti-seize on the O2 sensor threads, since:
1. I haven't found any when I unscrewed it from the old exhaust, and it's been there for over 30 000 kms. It came off easily.
2. I was afraid I'd muck up the sensor itself.
I torqued the sensor "by feel". So, turns out I did take some shortcuts.
Note for others, the Kawasaki O2 sensor requires a 17
mm spanner/socket, not the typical 22mm you apparently see more often for automotive O2 sensors. That makes it hard to get the special slotted socket, hence I used an ordinary 17 spanner. The previous exhaust had a 17-22 mm thread adapter.
It was a pain in the butt to simultaneously lift the exhaust from under the bike and screw it on without a second pair of hands or a jack, but sliding increasingly tall boxes underneath as I lifted eventually worked.
Thanks for all the help