Updated 6/8/13
To add my impressions to your wonderful write-up:
I received red silicone O-rings from IMSTO today in two sizes. Prior to that, I had only tried black 50A O-rings from Amazon and OEM keycaps.
The large silicone red O-rings from IMSTO are surprisingly large in person, but I found they work very well for those keys that don't have a cross-bar support, and the stem goes all the way up to the top of the cap. Keys such as zink keycaps and aluminum spacebar. They are just a bit too large for Ducky Shine II keycaps and reduce key travel dramatically. I can see these O-rings be very good for MX Black switches, where you don't need to push down much farther than the actuation point for any enhanced feel, resulting in less finger strain. (In my subjective opinion, I like the full travel distance for MX Browns, but MX Black with reduced travel distance is a benefit).
One thing to note about these O-rings is that they read their full thickness farther from their inner diameter, and thus farther from where they hit the switch bottom. It means that in some cases, they can actually do nothing as far as dampening the bottoming out sound goes. But they can be used in combination with a skinny O-ring following them, as substitutions for cross-bar support.
The skinny red silicone O-rings from IMSTO do close to nothing in terms of feel or sound for OEM keys that are not double-shots. They are noticeably thinner than the 50A O-rings from Amazon. Hitting a key with that O-ring hard still results in a harsh bottoming out sound, with a very slightly dampened tone. Very slightly. These O-rings may be best for Cherry-profile caps or DSA (I have a set of spherical DSA keycaps incoming and will test them soon). But I'm still surprised just how little these O-rings do for the bottoming out sound. ETA: just tried them on some double-shot keycaps with MX Blacks, and the dampening was better. So it depends highly on what kind of keycaps you have for these thin O-rings.
The black rubber 50A O-rings from Amazon: are my favorite so far. They don't result in a very significant travel reduction that impedes with my enjoyment of MX Browns, but they have VERY good bottom-out damping regardless of what kind of keycaps I tried. And even without bottoming out (I don't bottom out much), they dampen the key sound overall. I really like them for MX Browns.
ETA: