Improved sound quality, improved comfort, reduced ANC quality.
I originally purchased the Dekoni replacement pads through Massdrop in the Choice Suede material a little over a year ago. When I first received them, there was some initial difficulty in getting them to clip in properly to the XM3 ear cups. You just have to press a bit harder than you might be comfortable but they do indeed snap right into place.
Once installed properly I was really pleased with how comfortable they were on my ears, they generate much less heat (at least to me) compared to the stock pads but where I was happiest was with the slight adjustment to the sound signature. The XM3s aren't overly "bass-heavy" headphones, but in some genres it can be a bit overpowering. I found that these replacement pads from Dekoni do actually balance out the sound signature in a way that to me was quite pleasing.
So, improved comfort, improved sound quality, win-win, right?
Unfortunately that's not the end of the story. The replacement pads also dramatically reduce the effectiveness of the Active Noise Cancellation (which is kind of the point of buying the Sony XM3s in the first place). It lets in a lot more noise, which you wouldn't notice in a reasonably quiet space, but when there are voices nearby or perhaps dogs barking, it isn't drowned out in the slightest. It appears that sounds on the higher frequency range just come right through.
It's worth noting that this is referring to the experience with just Noise Cancellation turned on, not using the Ambient Sound mode.
So on one hand, these actually do improve the sound quality in a way that makes me really happy, they're also more comfortable on my ears / head for long listening sessions (such as listening to music all day while working from home); but they unfortunately cancel out the main feature and reason for buying these headphones in the first place. If having ANC doesn't matter too much to you then I actually do recommend these pads for the improved comfort and sound, but if you care about that Active Noise Cancellation, I have to say you should pass on these.
Would not recommend to a friend.