I just tried (a part of) this mod on the pleather pads I got with my HE-400. I'd been using the velours since I got the headphones, about a week ago. Though I loved the extremely airy treble and detail, I just couldn't live with the tizz/sibilance that just overshadowed everything; that and the lack of bass thump. Pleathers were all warm and muddy, not unpleasant but made my orthos sound like dynamics...
So, I mutilated my pleathers. But I didn't take them apart. After removing the inner ring, I calmly twisted the inside of the pads inside out, going around bit by bit and letting it go back to normal as I progressed, to make the underside appear and cut fairly large holes (larger than the original four, like 2-5mm) around the middle part of the underside, fairly close to each other (2-5mm) and following the 'circle' of the original four all around the pads. I got a pretty clean result actually, and the pads don't feel like they're dangling from the headphones.
It was a revelation. I love what I'm hearing now; fat (a bit too fat actually) low end and bass thump, a great thick but not muddy midrange, and a tamer treble (still has a lot of energy if the track has it). I lost a bit of air but not soundstage or detail, and yes the vocals are in a differnt league now. The evil high frequency tizz seems to be hidden (?) now, at least it's not the only thing I hear anymore.
My only question is: how do you think leaving the flaps, basically not ripping off the back part, affects the sound (only difference between the 'official' mod and what I did since you still use the ring and glue the filter back on; that and less holes for now)? The way I made those holes, they shouldn't be covered by the flaps or most of the mounting ring (outward holes would be covered by the ring anyway, right?), and I can still make more holes in the innermost part of the pads (easier that the holes I already made).