I have been running a straight through exhaust lately. Other than that I have done nothing to the engine.
For the first time ever, I have used a little oil, about a quarter litre in a couple thousand miles. Could these be connected? Thinking of popping a cat back on, just in case.
I suggest a pressure test on each cylinder.
If the pressure is down ( I have seen reference to acceptable pressure on here) on one or more cylinders then that is likely were your oil is going and putting the cat back on could then block the cat over time.
If pressures are even and acceptable then try refitting the cat and again monitor your oil level.
That should resolve the question.