These are my favorite:
http://www.amazon.com/dp/B0051XWXCE/ref=biss_dp_t_asn
I was thinking of getting these. That price compared to the WASD ones is crazy. The WASD ones felt harder than the others but the actuation reduction was quite nice.
-----008 Buna-N O-Ring, 50A Durometer, Round, Black, 3/16" ID, 5/16" OD, 1/16" Width
I have these, they were the original o-rings Geekhackers were buying a couple years ago, in the heady early days of o-ring mods, long before the "O-ring craze" caught on. They seem to have been one of the standbys that many people like over the years.
I'm currently typing, one type of o-ring on each hand on a Kinesis Advantage:
LEFT HAND/DOUBLESHOT KEYCAPS- 50A (008 Buna-N O-Ring, 50A Durometer, Round, Black, 3/16" ID, 5/16" OD, 1/16" Width)
RIGHT HAND/SINGLESHOT KEYCAPS- 40A-R (blue silicone from CPTBadAss...The WASD ones.
So there is a discrepancy with keycaps, but I can tell you this: With the 50A I feel a solid, damped, bottoming out. It kinda feels good, like a solid, quality piece of equipment. With the 40A there is less of a solid, damped bottoming out...the bottoming out feeling is minimized, it is less pronounced. Somewhat disguised. I'm not sure how to evaluate them directly, they are both good. I'm going to swap the o-rings onto the different key caps later this weekend and see what I think.
To compare the two...the 50A are excellent, no problems. The 40A are slightly softer, also excellent. I'm guessing, but if someone has RSI, I assume they would want softer...so the 40A would be better. I notice the difference when I hit the keys HARD for whatever reason. With the 50A, I feel the vibration in my other hand. Whit the 40A, I don't feel the vibration with the other hand.
As far as actuation...I'm not a hard core pounder of the keyboard, not very qualified to offer much input on the actuation thoughts at this point.
***Any o-ring performance is also determined by the type of keycap***