The Octane test has GI clamped to 1. It means almost zero caustics.
You can't compare 6 fastest gpus with a good dual Xeon rig.
Less noise, less heat, less power, less space.
The good side of gpu render is that we can replace our gpus. After 4 years of 24/7 of 4xGTX580s usage I can replace all of them with 4 Titans X and get 2,2X speed.
It's not true that the Titan X is fast as the Titan Z. Titan X is slight faster than the Titan Black, not 2X.
The bright side of gpu computing is the feedback. With Corona 1.0 we have a good rt feedback but not comparable to the Octane rt speed with 4 Titans.
I don't know if is possible to port Embree on gpus, I know that Thea did it.
The light distribution in Corona is better than the Octane one but the PMC (sort of Metropolis lt) produces incredible renders and we can use pmc always in realtime.
Never say never.