Nope, at best from 25% @10w to 35-40% @1 or 2w. Mars for instance use 600mA rated drivers with 460-540mA real current, at best 80% effiency. And since they save on heatsinks and prefer to use loud fans, the LEDs run relatively hot.
That's the main reason they burn out so fast.
For DIY one could use the cheap 3w Epi/Bridgelux diodes, rated for 700mA, and run them on multiple parallel strings to get the best out of it. At 70-100mA maybe up to 40% depending on wavelength/spectrum. I've seen people using lots of 3w full spectrum diodes in parallel to cover a 2' x 2' area.
I would use them only as cheap additional light to supplement white COB's with lower kelvins.
But if one want supplemenat light that can also be done it with a few Vero13's in 1750°K, which is a lot more efficient and is alot less wiring effort.
Whatever you do, it's never a really good solution. With bad diodes you can only build bad lights. Even if you can get a few percent over large quantities, they will never be able to keep up with good top bin LEDs.
