That said, if you're doing a lot of mutation of large JSON documents stored in a single Postgres row value, the storage/concurrency control behavior still won't be ideal.
Actually if you really need to update large JSON documents efficiently, probably the only really efficient technology would be ToroDB http://www.8kdata.com/torodb/
http://www.postgresql.org/docs/devel/static/functions-json.h...
That said, if you're doing a lot of mutation of large JSON documents stored in a single Postgres row value, the storage/concurrency control behavior still won't be ideal.