I've gotten mixed results using displacement in Vray, haven't yet tried it with corona. Usually, when I subdivided the polys a bit more, the displacement subdividing and presampling took less time than with a non-subdivided surface. the render was about the same. Also, lots of good displacement requires large displacement maps, that take a lot of memory resources.
With this much memory, you can try and subdivide the object as much as needed, use Displace with the map you were using as displacement, optimize it and collapse the modifier stack. I see you have lots of repeating objects in your animation, you could then scatter them using corona proxys. That shouldn't take much ram and you should not have any problems with displacement. Should also render faster.