> Not optimal efficiency, but better than Vorbis (which doesn't support coupling at all when there is more than two channels), and generally speaking good enough.
Vorbis is actually the only format I know that supports arbitrary multi-level coupling. It simply wasn't implemented in a real encoder until 2010 (and the fact that it took that long speaks a lot more to the demand for this feature than the difficulty). And of course, when it was implemented, it broke decoders that hadn't bothered with that part of the spec.
See http://people.xiph.org/~xiphmont/surround/demo2.html for details.
The most interesting part of that work, to me, was that the bulk of the gains came from re-training the VQ backend, not from the coupling itself. Opus uses algebraic VQ which does not need to be re-trained.