I'd have to agree. Even though you can power 3 6990s with a 1200 Watt unit you'd still be pushing it. It's good to leave a fair bit of head room with high efficiency PSUs. Often they operate at peak efficiency at ~80% or below load. You never want to get close to the peak load it can support because not only will you shorten its lifespan but because it will also pump out more heat due to inefficiency heat losses.
Atroxes, I like your signature..so I take it you've got yourself a incognito mining rig in a garbage can with a power line running to a nearby power pole?
Now way! Could it be! lol
To be honest I did not even pay attention to his avatar when I made that post.