Pattern noise eg. striping is in practice the greatest limitation to high ISO shooting, as I learnt from my my P45+; various tricks can be used to deal with this, also exploiting sensor features like the optical black edge pixels, but the Raw converter needs to help, and may need to be fed with calibration images at that high "ISO".
I also think reading the sensor slower, and possibly re-reading it, might also allow more data to be extracted.
If I may be allowed a dumb analogy, SLR focus experience has improved hugely for us geeks since user fine-focus adjustment was introduced, although the facilities were always there, to allow factory adjustment. The same goes for sensors. It's about exploiting hardware features that are already in place, it's about actually letting the user get all the juice out of the lemon, and the raw converter plays a huge role in that, fixing flaws, doing histogram striping compensation etc.
High ISO in practice is all about letting the user get the image, and making it usable even if resolution, color depth etc have to be sacrificed along the way, and the MF marketing guys and management just don't get this although the sensor guys do, because they work for the military, and all these image processing techniques are well known and can be dug out of the remote sensing and security industry literature if you have the patience.