GX7 with legacy teles
The ability of a lens to resolve fine details depends on its angular resolution, not on its focal length. Adding a converter does not increase a lens' angular resolution.
The Canon 200mm L lens has pretty good angular resolution so it is better for resolving fine details than many lenses that have longer focal lengths. However, there are lenses that have even better angular resolution - I'm told that the very long Canon L lenses are examples, but they are so expensive that I do not expect ever to be able to confirm that from personal experience!
I thought that it was interesting to find that some affordable lenses, like the 600mm Tele-Tokina, can do better, so I reported my experience. I should note though that the image contrast was much worse with the Tokina than with the Canon so it is not necessarily a "better" lens.
I used a converter with the Tokina lens because I did not know a priori what to expect from it, so using a converter is akin to using a magnifying lens for viewing a print.