cool test
are you using PT + PT or PT + HD cache?
Corona uses the same progressive technics as the most used GPU based kernels.
With 20 minutes you can't have this quality using one single Intel processor, the same is valid using one GTX780 or Titan.
You need more processors or a renderfarm.
Neither Octane can produce noiseless renders except you have 4 gpus and the final image is 2X the size.
Direct lighting is the best kernel for a gpu. Enrico Cerica create amazing renders using Direct Light AO and few lights, no caustics, path tracings, metropolis, etcetera.
I hope that this guy will enter in this community because he will be very surprised about Corona.
If it have Phi compatibility....everything will change.