The point of a teleconverter is to make your subject larger in the frame. The absolute best way to do this is to move closer to your subject. The next best way is to get a longer lens and if neither of those options work, use a teleconverter. Even with a teleconverter, there’s a maximum distance you shouldn’t shoot past. If you can’t see the detail when you’re shooting, it’s too far and you should move closer. There won’t be enough detail in the couple of pixels the small bird takes up in the frame to recover plus the teleconverter degrades the image and that’s really apparent when it’s still not close enough and the next step is to crop the image. I’ve found the #1 most popular reason for using a teleconverter, (getting “close” to really far away subjects) is its weakest use and all the aborations show up in that “extreme condition”.
Still, I use the 2x on my 70-200 and I’m happy with the results, though I’m aware of the compromise that’s required. I really like that I have 70mm f2.8 when needed or 400mm f5.6, (600mm effective in dx) using the same lens but I must be more aware of high iso and filling the frame as much as possible when the tc is attached. I can’t speak to the 1.4x or the 180-600 but I would suggest trying to move closer using the bare lens. Even taking away 1 extra stop of light from f6.3 could mean super high iso which leads to more image mushiness beyond what the tc will introduce. Fine if you can slow down the shutter speed enough to compensate and keep iso in check but a brick wall if you can’t give your image any more time, (like when shooting small birds and fast shutter speeds are absolutely mandatory).
Btw, one very strong argument FOR teleconverters is that it doesn’t reduce your minimum focusing distance; you can still be as close to the subject with or without the tc attached allowing nearly macro type shots that are very clean and this is where teleconverters are at their most powerful use case, imho. In other words, if you- using your lens/teleconverter and a fellow photographer- using a $15,500 600F4 with the built in TC had a comparison to who could photograph the shoelace on their foot, you'd win! The big guns cant shoot that close. But there's a reason the fellow photographer spent $15,500 on their lens; distance=$$$.
