Smokey may have pushed lean burn to the limits and likely obtained quite high overall efficiency but he didn't re-write the Second Law of Thermodynamics which applies to all heat engines. The high levels of nitrous oxides produced by the very hot combustion and difficulty in making a 3-way catalyst work would create more problems than it solved in today's terms.
Comments in the link you posted on the
eng-tips site are the ones I put the most trust in since from my membership there I know a number of those posters are highly-qualified and experienced automotive engineers. Plus it seems a couple of other posters have direct experience with Smokey's engine. But the fact that the patent hasn't been renewed is pretty telling of today's value of this design as a whole. And as pointed out, obtaining complete combustion is hardly a problem with modern fuel injection systems, and highly-optimized combustion chambers and valve timing.
Keys to efficiency for a gasoline engine - small displacement, high compression, minimization of throttling losses, optimum exploitation of the brake-specific fuel consumption characteristic, retention and re-use of waste coolant heat for quick warm-up - not coincidentally all items you can find today in a Toyota Prius.