Rawalanche, often you want to defend at all costs Corona... You should be more "unbiased" ;-)
Try to use VRay-RT in DR mode and, you will see with your eyes, there isn't so big difference (viewport feedback) if you mode camera, object or change color, shader, etc..
Ok, if you merge a 10.000.000 poligon object, you have to share the new mesh over the network and wait a bit. In local mode it will be faster, obviously.
I remember a video where a guy used 10 nodes with VRay-RT CPU. Very fast and super feedback.
As some user already say, also Octane has a very good IR DR service. Why not Corona?
With IR DR, you will not loose interactivity performance. Just look at VRay and Octane.
And, for me, it's an high priority.