I happen to have five of 2x tele-converters fit Konica AR mount in hands these days. Lined them up in front of a Konica camera, guess what in its viewfinder?
The converters should make the focusing range 10 times longer (5 x 2x; 50mm to 500mm). However, the focusing distances match the distance marks on the lens. The downside is that it's very dark in the viewfinder. I had to point to a bright light to see the image in the viewfinder.
That's the effect. Practically, it's not much useful.