Here is the thread here of the guy that made the video, and you can find some impressions there. In my experience, although I didn't A/B them, the bass tightens, but the quantity might be slightly less, and the soundstage and imaging improve a little and overall everything sounds clearer, but take that with a grain of salt since it has been a long while since I listened the HD555.
It is easy to do, just be careful of course. Don't drop the screws anywhere. Try not to flex the headphones too much. There is nothing to worry about with this mod. Even if you think you didn't hear a difference, there would be no benefit of putting the foam back in. I personally found the bass to sound a tad bit more natural and full, just sounded as if the the speaker moved air slightly better, which makes sense. Its not a huge difference, but I felt good about it. I had listened to music for about an hour just before, then went back and listened to the same thing.
Ok just finished the mod. First thoughts?, no that much more bass..... Then again I have my Shure 530 which have a ton of bass so I can't be a great judge at that. What I can say for sure is that it is best enjoyed at lower volumes after the mod. When I cranked it up past oh mid way on the second gen nano, the highs and bass became muddy.. It sounded too "open" and airy. I'll give these some more time but if it sounds like this after a week, Im putting the fome back.
I see the comments that say the mod does wonders, are they just nubs or am I just a audiophile now that dislike muddy base, hahah (damn you ety's!)