You don't have to convince me about importance of optimization, i have 6 years old computer with 16GB RAM, so i'm quite familiar with situation when scene doesn't fit into RAM :] But i think it's quite important to optimize in smart way. In this case lowering texture's resolution would be smart optimization, as it makes very little visual difference to the end result. On the other hand, lowering bit depth, would be... aaa, just optimization.
P.S. not sure what that video has to do with our discussion, but why are you so sure that Corona can't render 20M trees?