Interesting, I had once made this same effect, but then questioned its efficiency vs. a canvas with grab layout. I settled on the Canvas. Have you tested it vs a canvas for full-screen post processing effects?
Nice question. I just made a quick test with a 1024x768 window and got almost the same results with canvas and Apply Below. Actually the effect gives me 1 or 2 more fps, but this is negligible. The canvas also adds a little more vram, probably because it has to store its original texture. So technically, even if you stretch a 1x1 canvas in the entire layout it still will use a tiny bit more vram. Nothing to worry about.
Based on this: just use the one you prefer most. I personally think the effect is simpler to use though.
Anyway, I suggest people to make more tests, probably there's cases where this could not be true.
@The_Funny_Guy: I advise you to make it through events or a plugin. An effect would give lot slower results an unnecessarily narrow your possible audience to people with good graphics cards. It's so easy to implement this through events that doesn't worth it. It also would get pretty ugly near the window borders with intense shakes.