Twenty years ago I tried this mod, because the HD600s did then indeed sound "foggy". But quite soon I put the foam back mainly for the same reasons as you. I then sold the phones to a friend.Initially it seemed like a nice mod. First observation was of them being a little more energetic, more aggressive... but they lost their characteristic sense of ease. More listening indicated other stuff they lost too. The whole sound became thinner, leaner and less impactful. The highs became starker but the sound as a whole lost some definition.
But two months ago I bought another pair, the 2019 version. They do not sound foggy at all.