I have a WASD with browns and the red 0.2 o-rings. They do help with the noise, and the shortened key travel and softer bottom-out is nice. I like them.
I haven't tried 0.4 rings, but I tried doubling up the 0.2 rings for a similar effect. Short version: I don't like it. When typing, the sudden bottom-out is jarring. It feels like typing on an Apple-style chicklet keyboard. It seems better to allow some travel so your fingers can decelerate naturally under resistance from the springs.
I still kept doubled rings on the modifiers and the arrow keys. Holding down shift is easier, and the short travel one the arrow keys makes tap-tap-taping them quicker when navigating text.
If you're a gamer and need to tap keys repeatedly, thicker o-rings might be nice. But not for general typing.