Okay, so I received the Shure 840 pads today and installed them on the 7520. Installation was a breeze, they're slightly larger than the 7520 pads in almost every way.
In terms of sound signature I'm not noticing any immediate differences, although I think it filters the highs every so slightly because the fabric that covers the drivers is slightly thicker.
Comfortwise my initial impression is that it's very comfortable. The 7520 pads are about 15mm deep, whereas the 840 pads are a bit over 20mm deep. Which leaves a bit more breathing room for your ears.
Will report back after some more testing though.
Edit: here are some pics.
Edited by starfly - 11/21/13 at 9:11am