Turns out this was the front catalytic converter.

Error codes didn't directly indicate this, but an occasional exhaust rattle/boom plus misfire codes from cyl 1-3 gave the diagnostician enough to figure it out.

Power levels fluctuated and it was just about drivable because the exhaust was only partially blocked on 1-3, so it had 3 working cylinders at worst, 4-5 at best.

Hope that's useful for anyone who comes across this type of issue in future.