Random defects aren't related to GPU revision. TrueAudio isn't official part of HD 7790's feature set, so AMD could sell Bonaire GPUs with defective TrueAudio block. Even single- or double-digit numbers of such GPUs on market would prevent AMD from enabling this functionality later.
True, but there's nothing pointing there would be any out there. Of course everything is possible, but I think someone from AMD (Dave?) did hint that it would come to 7790 too via driver update. Not 100% sure obviously though, it was quite some time ago.
Also, AMD has enabled previously features that came with new cards on older hardware, too, if the hardware supports it.