Thanks, I was so tempted to try the mod I did it the day after posting my question :D I also did this mod later to a fully burned in pair of HFI-700 (I don't think they sound that sibilant compared to a half-way burned in HFI-780 but still, out of curiosity, did the same mod to them). It doesn't matter which hole is covered, the result is the same.
It's amazing how many people don't consider this mod. I come from a Grado background, so modding is no new territory. Besides, this is just the Grado hole punch mod Ultrasone style. I think they do this mod to the ATH-ADX00 series too.
I just did this on my 300+ hour 780s after I stuck some DT250 velour pads on them -- the bass suffered! Naturally I had to try this mod, and it brought my bass back! But even without the velour pads, I felt like the bass of my 780s had settled down quite a lot after burn-in.
I hope this mod gets more exposure, given the popularity of the Grado hole punch mod.
I much prefer pleather over the Beyer velour pads as the change they make is rather too much to sacrifice for their comfort, although some might say they never notice any changes switching to velour (and personally I don't find pleather uncomfortable). If you don't have problem wearing pleather, switching back to the stock pads can make the bass and overall PRaT even better on the 780.