MOST IMPORTANT: Get Corona to render different noise on every rendering -->
then you can later merge the pics from different maschines into one cleaner pic.
Stone-age old trick for hand made DR.
Achieve this by submitting still-scenes as short animation over a renderfarm.
Than mix the frames in a 32bit capable App like Photoshop, Nuke, Fusion, AfterE.
Automate it somehow for faster workflow.
I don't know if u can force Corona to render the grain differently on every render, just testing Corona for 2 days.