I just installed 40 durometer silicone o-rings, standard 008 size as above.
They are much softer than the rubber ones, and a rust (red) color. They were also much easier to put on, you roll them on the stem with your fingertips and they snap into place at the end. It took me about 30 minutes to do the whole keyboard.
My arrow keys had smaller struts than the other keys, so they would still clack a bit when bottoming out with the o-rings on, so I stacked the o-rings with neoprene 50 duro washers (1/32" thick, 7/32" ID, 7/16" OD, larger OD would fit too). Depending on your keys, if they have no struts and still clack, you could try stacking multiple washers, o-rings, dental bands, whatever you have lying around, to see which works best. Pic showing the new o-rings, as well the washers, and a standard black rubber 70 durometer o-ring.
Verdict: less noise, softer landing than standard rubber o-rings. Key travel is reduced by around .75mm, there's still ample travel left after the key registers.
The standard o-rings reduced the travel by about 1mm, I think because they didn't compress at all during typing, and was harder for the keycap struts to dig into them.
The silicone o-rings were a bit pricey, 10 cents each (same price as the soft landing pads -which I haven't tried- from elitekeyboards.com). The black rubber o-rings were only 4 cents each. However I expect the silicone o-rings to last a very long time, they seem much more durable than dental bands, dog grooming bands, etc. This mod is definitely a keeper for me.