I've had the Dekoni memory foam pads for a while now and recently switched back to the stock pads just for kicks and to see if I could notice any difference.
The stock pads definitely have a more open/spacial soundstage but produce what I'm now going to label as limiting bass. The memory foam ones produce richer and deeper bass where it matters most. The side-effect of this is that the soundstage is brought closer, so slightly less wide. This is the only difference I notice, and this isn't a bad thing either. It's not better, it's not worse, it's just different. I'd say it's like the difference between the Fidelio X2 now vs say the stock HD650s in terms of openness vs bass and soundstage direction.
The upshot being the HD650s with these memory foam pads produce a wider soundstage than the X2 ever did and the bass is rich but more neutral whereas the X2s focused too much on deeper bass.
I think these pads were a great investment, I've put them back on and enjoy the more musical intimacy with them, especially with acoustic and jazz music.
As far as I'm concerned this combo for me is end game. I know I could get better if I spent more, but I really don't need better, I'm just enjoying this for pretty much everything. Perhaps the amp combo has something to do with this as well, but it's a great match for my taste and I can't see that changing for the foreseeable future!
Just hoping that Sennheiser never stop servicing the 650s going forward