The 30-40% figure does not surprise me in the least for this reason. I hope others will take the time to do their homework...
We have been graced with a number of real experts and engineers here, including our VT-based friend nofossil, who has developed products for boiler monitoring. I remember some thread where he detailed the actual delivered efficiency of his system - which was an indoor small-firebox gasifier along with storage and top-notch controls. I think the figure he calculated was 56%.
It doesn't take much of a stretch to look at an outdoor unit and see:
Larger Firebox
less in terms of monitoring and control
Heat losses from the colder ambients
"1st generation" engineering (learning on the job, so to speak).
My guess would have been about 40% compared to Nofossils 56%. That's not too bad, really, when you figure that the older models of those same OWB's are probably 20-25% at best.
I agree with heaterman and the rest of you who say we need a single standard. As of now, we have the American Way (PT Barnum), which unfortunately means stuff like this:
http://fisherstovesusa.com/