I think the first mistake made by many is that people are expecting the HD800's to be perfect, or to be for everyone. They aren't and they won't. Sorry if that is what people were expecting for $1400, but absolute perfection doesn't exist in the audio hobby. No one will ever agree collectively that one headphone is perfect. There are much more expensive headphones that aren't perfect.(L3000, HP1000, etc. the list goes on)
Anyways, with that said, in regards to sibilance I don't find there to be an issue(although that is just my ears, it could very well be). In regards to the "hottness" of the highs, I do find it to be an issue on some tracks. For example, the background music of different Michael Jackson and Rick James songs are too bright to handle, and thus I find myself listening to them less. This is a bit weird as there are plenty of songs by Journey and Mariah Carey for example, that have just as much emphasis in the highs, but are not bright or too bright to my ears.
This leads me to believe it is about the recording, but I don't know if that is the best way to decide for myself. But no one will be able to say definitely. What could easily be classified as an overemphasis in the highs on some songs, can also be classified as a revealing of less than perfect recordings, or as an under-emphasis of the highs with other headphones. The only person who can say for sure, is the person actually recorded it. Outside of that person's opinion, our musical preferences are what are guiding our decision of "right" vs. "wrong", so it is a crapshoot as you see by the differing opinions in this thread.
Trying to decide definitively whether or not this is an "issue" with the HD800 is relatively pointless imho as it is too impossible to say for sure. There are simply too many variables. But what I can say, is that without a doubt, some tracks are "too bright". Of course it is a small percentage of my collection, but I do LOVE those tracks and now I DO listen to them less.
We can argue all day about whether or not it is the recording or the phones. The bottom line is that using these phones, there is a possibility that they may be too bright on some songs. Regardless of whether or not it is the phone or the recording, the fact of the matter is that I hear it on some songs.