I farm Ony10/25 and VoA10/25 weekly on my group. I don't think I could manage it tanking and running DPS. I stick to 4Ele+Boomkin and let other people worry about tanking/healing. If you can box Naxx, you can box Ony/VoA. Both are much simpler fights.

I've 5boxed ToC25, barely scraping through, still learning how to deal with the Icehowl's charge and movement during Anub.

Older content you can make mistakes. In fights like the Beasts/Twins or VoA's Koralon, you really have no margin of error. If you don't move/switch in literally 1-2 seconds, you die.

When I first started boxing on my server people wouldn't invite me to anything. I had to solo tank Ony/ToC a few times (and win WG for us a few times, nearly soloing the keep) to get to know a few people and then talk them into letting me bring my shamans next time. After a while, people actually send me whispers now asking if I'm locked to Ony/VoA yet, because they know I know the fights and it's a fast way to fill a solid group on a fairly low pop server. That's why I really recommend going with as simple a group composition as possible. You have very little margin of error and the less you have to think about the less chance you'll make mistakes.

It would be nice to have a guild that isn't biased against raid boxing.