The answer is a definitive "We don't know"
But most likely things will transplant. The WRX and the RS aren't even mutually exclusive. Many things will go between. Some things that, in all probability won't fit are pistons and camshafts. One thing that won't be easy, but possibly doable is are the heads. In all likelihood they will bolt together, but the AVCS won't work w/o some extra hardward...