I have done some more research on how different quality images are sent to different device types for the same book. For this test I used
Wafu Cool Japan Guide by Japan Publicity. It is composed of large color images and is free. I chose a representative page, titled "Where to Go?". It is the fifth page (counting the cover) and contains a map of Japan. The results shown below are: image type, dimensions (WxH), file size, and quality of the smallest text on the map.
Test results:
AZW3 download targeted for a Paperwhite II: color JPEG, 925 x 1181, 119 KB, just barely readable.
KFX on Paperwhite II: gray scale JPEG-XR, 654 x 834, 76 KB, slightly less readable than the AZW3 version.
KFX on iPad Air 2: color JPEG, 1504 x 1919, 776 KB, very readable.
KFX on iPhone 6: Identical to the iPad.
That is as far as I can go using the devices I have on hand. I am curious what the results would be for other devices.
If anyone with a higher resolution e-ink Kindle (PW3 or Voyage) wants to contribute they should get this book both as an AZW3 download from Amazon.com and as a KFX delivered directly to device and see what image formats are present.
I am also curious about the Fire tables. I believe that they also support KFX, but I don't know how accessible the document storage is on those devices.