I got 2.5mm 40A O-rings to reduce the noise specifically on the 4 buttons I use for Mania, since I was getting complaints about the tapping being too loud.
Doesn't feel much different, but keys I put O-rings under are definitely significantly quieter when pressed rapidly compared to the non-O-ring keys.
I'm fairly sure it depends on the softness/size of the O-rings you get though. If they're too hard or you put multiple under your keys, you're going to find it takes much less pressure to register a key press.
Though some people might find that a good thing for Mania.
In the end, I use a Corsair keyboard and the O-rings definitely did their job without affecting the feel of the keys too much.