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[Jwood]

Or here could be an Optimizer 250.
260/2000=.13 x100=13 so 13% is going out the stack and 87% is being transfered.
Even if none of that makes sense, it sure sounds good to me 

[Sprinter]

Forget relying on some company, DIY!!!!  One of the first things I look at on a call is stack temps. That's my first clue on wether it needs a cleaning or not. But there are so many different brands and they all have different temps. I think the consensus of members and their own findings is way more credible than what some OWB manufacturer "claims" in a lab .... notice none of them list this anyways.
Its a lot more than a couple probes and calculating efficiency. Weight of the wood and Wood MC will give you a net BTU input and then measuring how much BTU you are actually using in the home. You'd need to know GPM , radiant emitters and DT to get rough estimates. This is where the guys that have storage tanks have a known load that is easily calculated when all other loads are off. There are several good threads on calculating this stuff on hearth, and charts with stack temps over wood in lbs over wood MC equals thermal efficiency plus or minus 10% for example.
Either way is good stuff to know about your system. It's always a good idea to monitor stack temps if you can.

[slimjim]

Sure seems like a good start to me, it's simple and would seem to be hard to manipulate, would the customer care enough about it to pay for the pyrometers and recording devices that would have to be left in the field, I would like results from the field rather than a lab that is very controlled, take an average of many units rather than a single test,

[Jwood]

I agree sprinter you can probably make it as complicated as you want but every wood burns different but with those two probes you could periodically check the difference any time you want. Let's say 20% moisture oak gives you 1200 degrees in the fire box and 550 in the stack, then take 30% moisture your fire temp is going to go down but I would have to assume your stack temp will drop as well. The boiler is still extracting the same amount of heat before it exits the chimney, again I'm not saying this is correct in any way that's just how I see it I guess.

[TheBoiler]

You loose efficiency not just at the chimney, incomplete combustion, heat loss elsewhere.

Off the top of my head I would think the easiest way is to put in a load of wood of known energy (weight and moisture) and use it to heat a known volume of water. Measure the temperature gain.

The tricky bit is when you have different methods of operation ranging from full burn to smoldering.

[Jwood]

Ok well I was smart for a few minutes anyway