Hi SG,
Not sure I can answer your specific questions with any degree of accuracy due to not having the practical experience of 16 bit pipeline being a Windows user myself. But I have some opinion FWIW which I suggest you should take with a pinch of salt until such time as you are able to prove yourself
...So does that mean, at least with Windows computers since Vista, that Canon with their XPS drivers could produce a broader gamut than Epson, and smoother tonality (less banding) too?
AFAIK the Canon XPS drivers have to be used in conjunction with the Canon applications Canon Print Studio Pro plugin or Canon Easy-PhotoPrint Pro to use a 16 bit print pipeline. Although I do have the XPS drivers and a couple of the cheapest Canon inkjets the Canon software is not available for either to allow 16 bit. I can print via the XPS driver and although slight difference in the results to the standard driver I do not believe that this is 16 bit and the difference can be explained by just the fact that the driver is different.
I do not think that you would be able to produce a wider colour gamut in 16 bit v 8 bit that would be the limit of the printer inks and paper combination. In theory it should be possible to see a smoother gradation between tonality changes with 16 bit. I have only ever seen one example that
might have demonstrated this but it was very subtle and unoticeable at 'normal' viewing distances plus the test condition unclear therefore I would not want to draw any conclusions
And what happens with Qimage Ultimate if it is still older 8 bit software feeding into a XPS 16 bit Canon driver? Might be reason Qimage isn't ported to Apple since they have been using 16 bit, although unsure if Epson has bothered to upgrade their drivers to 16 bit for that OS.
Again I have no real world experience of Qimage but from memory the author did not support 16 bit due to lack of support in Windows and I suspect that he would have had to actually write the 16 bit drivers. Plus again as I understand it was felt that 16 bit printing brought nothing to the table. Might be worthwhile contacting Qimage to see if anything has changed?
My impression so far is that Windows users should not be unduly concerned about not having a 16 bit print pipeline readily available as nearly all the expert comments I have seen state that it is unlikely to bring very much to the table in terms of image quality for photographic work