A 100% monitor view is not the criterion for TC feasibility. The subject, downsampled to the same size in pixels, or at the same size, printed, is the real criterion. You don't need full original pixel-to-pixel contrast to get more absolute subject detail and contrast.
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I know there are differences between prints and enlarged views of crops on the monitor which, even at just 100%, might represent a 36" wide print which you might never make. However, I would have thought if the differences on monitor are slight at high magnification, they will also at best be slight, but more probably non-existent, on print.
There is an issue as to whether or not to downsample the larger image (with TC) or upsample the smaller image. Downsampling the larger image involves throwing away information. Upsampling the smaller image involves retaining the information. I prefer to upsample for this reason. However, it's perfectly legitimate to downsample the larger image, and even both images, for the purpose of gauging how the actual print sizes might compare.
The difficulty I'm having is in understanding how things might improve for the TC image, relative to the smaller image, when one or both are downsampled so they are the same size on monitor, and/or print. As both images are reduced in size, any differences between them which might have been visible at higher magnification, will also be reduced.
I would have thought, if you really want to assess the benefits of using a TC, you should upsample the smaller image, compare both images at high magnification to first determine if there really is any more detail at all visible in the TC image. You might be surprised to find that sometimes there isn't, in which case there's no point in using the TC whatever the intended size of the print.
Having determined that the TC image really does produce more detail, you can then downsample both images to determine at what print size such differences become irrelevant, because they will become irrelevant at small enough sizes.
I generally find that the chief reason one can not be definite about the benefits of using a TC is because the results will vary greatly depending on the contrast of the subject. Generally, low contrast and distant subjects benefit the least. High contrast and fairly close subjects benefit the most. Even a cheap zoom with TC will reveal more lines on a test chart than without TC.
The bottom line appears to be, whatever the quality of your lens, adding a TC will make it an inferior lens but of greater focal length.