Well, I did the test, as best I could. I went to a local school with a great brick wall, put the D800E (at ISO100) and 70-200 f/2.8 VRII lens with an RRS plate on its foot, on a Gitzo 3 series carbon tripod on an Arca-Swiss B-1 head. I leveled the camera and looked through the finder with the 1.7 extender on, (340mm). I placed blue tape at the corners of the image with the camera at 340mm. I then made exposures with a cable release using mirror lock and waiting at least 15 seconds between mirror up and firing the shutter. I focused prior to each exposure using the center spot with the AF button, not the shutter release. I think, hope my technique was pretty careful. I made exposures with the extender at f/4.8 (wide open), f/5.6, and f/8. I made similar exposures without the extender on using the same technique detailed above. I went home, and downloaded them to Lightroom 5.3. I adjusted the exposure and white balance to even them out a bit but made no other adjustments, no sharpening, clarity, vibrance, you get the picture (pun intended). I then cropped them to the same size. Here they are. To me they are so close that I'll likely not use the extender. If anyone would like, I can send them full size files to play with. Since I can have only 4 attachments per post, I'll reply again and show the full wall. This crop is VERY SMALL. You'll have to look carefully at the full wall to see the area in the center. If anyone cares... and has a suggestion for a better way to do this, I'd love to hear it.