Well the K92s have just arrived.
Isolation wise they aren't quite as good as my HD280 pros (not HD250 pros as I wrote in my post above
) but they aren't bad. The HD280s provide a lot more pressure on the ear cups to really clamp them on your head, whereas the K92s are overall a lot lighter built, with thinner plastic shells, and there's less clamping pressure. Which means that they'll be more comfortable to wear for longer periods but won't seal quite as well than the HD280 Pros.
Sound-wise I'm currently not impressed with the K92s. There's a lot of bass (they do go lower than the HD280s) and there's a very 'tizzy' high end with no body at all to cymbal hits. All the mids sound very muffled and pushed back in the mix at the moment. Which is not what you want for vocal tracking headphones.
But they are AKGs, so they are the only headphone make that do need a burn-in period. So they are plugged in on repeat and I'll give an update in a couple of days. Currently though, I wish I'd paid double to get some more HD280s which sound far more balanced and do provide that bit extra isolation, which is important when trying to cut out bleed into the mic, especially if you have a quiet singer.