IF Canon did it right you get free image stacking (4 images) which should result in less noise and better color reproduction
^ This is the crux of my question: if I don't need the full sensor resolution anyway, maybe doing (some of) the downsizing ahead of time results in a better final image. It all depends on the specifics of how RAW and sRAW actually map sensor sites to the file.
If you need a simple answer: I does not.
Relatedly, does anyone know whether Canon's RAW format stores post-interpolation RGB pixels, or does it separately store the red, green and blue data as pre-interpolation discrete readouts? If the latter, whatever smarts could have been applied to the sRAW should still be applicable to the full sized RAW image.
The RAW format records non-interpolated RGB values. Yes, whatever Canon is doing to produce the smaller "RAW" formats, you can do it yourself, if you knew how (and if you find a really good reason for that).