Good points @dylan.christopherson, thanks.
From what I understand, quantifying stack temperature would only go so far though. Assessing that and O2 content of the combustion gas could give you the combustion efficiency but that wouldn’t really translate to heat exchange efficiency, especially for a small residential system. I will get out my Fluke though and poke around more on the stack.
That makes sense in terms of the goal with space heating: first transfer heat from oil combustion to glycol; then transfer heat from glycol (via radiation) to the air. So the lower the glycol return temperature, the better the heat exchange to the air has been. That doesn’t, though, quantify the efficiency without a lot of hard-to-get data.
What I’m thinking is: Looking at your house and doing some comparative math (above) I see you are perhaps 3X the raw efficiency. Obviously a lot of that has to be due to the better sealing and insulation of your house and I suspect (?) you know that due to your relative household efficiency given to you by the gas company. Maybe accounting for a more efficient gas boiler and your heating practices (not conditioning the whole space for example) we could reduce it to, say, my boiler being half the efficiency or within some believable realm. The trick is where to attribute the various inefficiencies when nothing is exactly quantified!
I’m going to start on something I can wrap my head around: the hot water supply. Standby.