Tonight i decided to do it . You prolly will not believe, but wideness of soundstage and overall airyness is definitely improved. Significantly. I guarantee you that you will hear the difference! Just play Radiohead's How to dissapear completely before and after modding
In fact i've made original zbwu's mod but since my cans were already hwc's foam modded i've combined those 2 mods in one - i've cutted (with soldering-iron, no less) the inner grill and foam'd the remaining surface around driver chassis. Plus i've got some tips how to make this mod safe and not ruin cans' look. Cause it looks kinda weird when you see the driver, acoustic screen and wires around it when you have only outer grill. The driver itself and screen are kinda white, so you can see it clear through the black outer grill. So i painted the screen and driver's back with black marker and now it looks simply beautiful throw the black grill. Too bad the camera can't catch how it looks correctly, but i'll post pics anyway.
Second, there's a hole on the other side of the driver which goes throw entire driver so you can see the membrane from the back side of the driver. Now, when you don't have the fabric covered inner grill that protected the hole to membrane from dust, etc you gotta protect it. I've used that fabric from the inner grill to cover it. Be careful with glue here, one blob on the membrane through that hole=you have no cans anymore%). This applies to whole procedure, look what you doing, don't cut off necessary stuff (like screws' holes) while cutting off the grill. And don't mess up the housing while operating the soldering-iron.
P.S. In original HD555/H595 mod thread there was a suggestion that cutting off the inner grill reduces the strenght of the headpnone. Well, that's is not true, the inner grill is made of very soft plastic, you'll see it, so it don't reinforces anything. One thing is reducing though-dust protection...but that's all.
P.P.S. While that mode can be done in half-hour, i've needed around 3 hours to make it all good and clean. If you wanna do it, make it the right way, no rush, and i guarantee - you will not regret.