Just tried the extension under Mac OS 10.8.5 and Safari 6.0.5.
I'm using the Magic Mouse and scrolling with the middle scroll area on the mouse. Flicking up to the top of a pace (extensions.apple.com) still bounces the content when it reaches the end.
So, here's the deal. Any pages that you open from a previous session will not have the bouncing disabled, but newly loaded pages will have the bounce disabled.
It's a little detail, but nice to know if you install the extension restart Safari, reload your previous session and still see pages bouncing.
Still, it's a shame that we have to disable the bounce through CSS, and can't set up an NSUserDefaults to never ever bounce a web view, scrollView, or whatever on the Mac.
Thanks Lloeki!