I did this with no expectations, only to see if anything was possible. I won't show 100% crops since they won't stand up to scrutiny. These are full frame, resized for the web with default screen sharpening in Lightroom. This is not a competition, this is only an exhibition. Please.....no flaming.
Northern Cardinal
Cedar Waxwing
Ruby-throated Hummingbird
American Robin
Rose-breasted Grosbeak
These birds were a long way off, perhaps 100 to 150 yards. The hummingbird was barely visible to the naked eye. All were manually focused, although AF could have been used when there was sufficient light. I find AF with the converter slightly annoying, it's a personal thing. A remote release is definitely needed for any kind of useable results.
If everything came out blurry and useless I wouldn't have been at all surprised, but these are better than that, I think. I don't know how these would compare to just cropping full size images. I understand the adage of "get as close as you can and then get ten feet closer." It's very true. My wife and I are the rankest amateurs at birding, and many times the camera is there only to get some image so that we can later try to identify what kind of bird we saw. I think these images would serve that purpose just fine. At 1020mm (1530mm in 35mm equivalence) they're definitely the longest focal length I've ever shot.
So go ahead and rip my technique to shreds. I'm putting these on here for anyone who's ever pondered about jury-rigging a long combination.