Not many I guess. T1.2 sounds different than T1.1 no matter what damping material will be used. The deal will be with midrange, which is more distant and warm in T1.2 due to pads and slight changes in baffle material.
I've tested your modification on T1.2 and the result is very priomising, thank you for the idea. But unfortunately whole piece of cotton petal is affecting the sense of airness, making them sound slightly less three-dimentional by simply "closing" headphones that way. In some systems the most optimal way in my opinion is to use cotton petals with hole - the same way as Beyer used over the years with vide range of their DT series. You can tame a little resonating highs that way, keep airiness and warm midrange just a bit. The results and the whole point of using this modifications depends on owned gear. In some situations whole petals seems to work better, but still there is negative effect on soundstage and "magic" that T1 have. I think this is caused by messing with interior of the earcup itself. When it is empty, it works like acoustic deflector and WITH conjunction with driver. Those two elements works as a whole and I think it should be left in stock form or modified as secondary.
Since Beyerdynamic put a suppressor on central hole of the driver in T1.2, I would suggest to play with it. In both revisions. I made an
"EvilKillaruna Mod". Before I've sorted things out, I've beet trying to put there Westone TrueFit with removed sleeve, then exchanged them for pieces of yellow ear plugs used for hearing safety. RCA caps can be used too if there is no other option available. The point was to completely close that hole in both drivers. It helped with excessive amount of highs without obvious negative effects (at least in T1.2). Still I missed some magic and sparkling together with holographic midrange however.
Then I tried to go back to Westone TrueFit, but
this time WITH sleeve and much more shortened, to avoid closing the suppressor inside each driver completely. Each TrueFit was shortened to about 4mm height. With new T1 you can achive for now the best results with small Westone TrueFit tips with green (or blue after shortening) sleeves just inserted as they are directly into central hole of the driver. Blue and green TrueFits are the 1st and 2nd from upper row:
As I said before, it is good to shorten them to the point they won't make contact with mesh inside the central hole of the driver. With the first revision of T1 I would choose bigger black tips, because of bigger diameter of the hole in each driver. TrueFit should go inside the hole like this:
Effects: Midrange is now properly focused and highs are not pumped up so much, yet soundstage with holography and sparkling was kept mostly as it was before. No need for any damping material since Beyers drivers are tuned for using interior of the cup as acoustic deflector and this mod has probably proven that.
I am so far VERY happy with the results.