deskthority

I'm more interested in travel reduction than cushioning. So I want the thickest hardest O-ring on the market. What should I get?

You could look into getting a 70A hardness rubber o-rings. This was the first site to come to mind http://lpwl.bigcartel.com/heeeelp . I think the owner of that shop comes on DT every once in a while as well.

Is the thickness listed the amount of travel reduction or something else. For example a lot of the cherry o-rings for sale list either .2mm or .4mm of reduction. If that's the case these offer a lot of reduction.

Well depends on the keycap more then anything, if the keycap hits on the top of the keyswitch housing then yes it should reduce the travel by the thickness of the o-ring. If that is not the case then it will most likely be less (if any reduction at all!)