The point of a tc would be to get "more reach" ie fill more of the frame with the subject. IME using tc's with slower lenses more often doesn't pay off even if the tc works well with the lens and iq holds up and you are able to compensate with good technique for the slower shutter speeds and increased effect of shake and vibration. Most often the results are little better, if at all, over increased cropping and/or resizing. The more effective way to get a bigger picture of your subject in the end is a camera with more pixels IMO.
I wil qualify that by saying that this can be a bit subject dependent.
cooltouch did a study on mflenses stacking tc's on a quality 300mm f2.8. He noted that although the images looked increasingly terrible, when processed and resized it was clear that the stacking had increased the information in the image, specifically the legibility of writing on a small sign. I can suggest that if your subject has fine detail eg feathers, thats what's lost first and tc's are less beneficial. However if eg ships or aeroplanes are your thing then a tc could be more beneficial.
Last edited by marcusBMG; 12-08-2019 at 06:30 AM.