@CobKits, Yes, I'd put more trust in replacing them than in doubling them up. I'd really have to "triple them up" already to compete with current COBs/led strips.
With pre drilled heat sinks you'd only have to replace the COB holder if the COB has a different size. Perhaps slightly less convenient than simply replacing the COB, but not that much of an issue either.
It might not move as fast again as the last 2 years, but I'm pretty sure from now to 3 to 4 years in the future the same step forward will have been made. These things will start hitting mainstream more and more and prices will tumble further. In a sense that's also the equivalent of increasing the efficiency.
Technology will shift too. Right now I guess I'd rather go for low to medium intensity led strips than COBs (or boards). Much easier to turn into a fixture with little cost in cooling and better light distribution. Strips also seem to be coming down in price even faster than COBs.
Not sure if that will ever become a viable option, but as much as people seem to detest purple lights here, a purple coating on a Citizen COB does offer some extra efficiency too. They are just way too expensive to make them worth it. That could change too.