Thanks eidy!
There was indeed some space for the changes that you've suggested. Ray epsilon went up to 0.00025, max depth - down to 5 (the light losses were small enough to be partly compensated by raising the lamp power). Unfortunately anything beyond those values caused some visible quality issues.
I also noticed that there is no need to set the max samples value to more than 250.
All this saved over 30 seconds of rendering. The time is now just below 1:30 on a Quadro 4000.
Cheers,
Szymon
i7 5960X | 64GB RAM | Win10 Pro | ASUS X99 Pro mobo | Chieftec 1250W psu
MSI GTX 780 Ti OC 3GB | 2 x GIGABYTE GTX 780 OC 3GB on USB risers | MSI GTX 750 Ti for display
Blender + Octane | C4D + Corona
Flickr profile:
http://www.flickr.com/photos/kubicki3d