Nice test, thanks for posting. I wasn't fully aware that the complexity of a scene does affect the compression so much, i thought the files will just end up bigger when there are more details in a scene. Now it seems it's more a compromise between increasing file-size and getting softer. Good to know.
10:1 and so on will still be fine for less complex scenes, especially if your target is HD and/or the web... So don't panic.
Looking forward to your next test John, pretty girl shot on a prime sounds like a good setup for a compression-test. ;)