I've got nothing to add to the OP's question, however, in the interest of curating data points on TC use...
I have 2 67 teleconverters, the 1.4x and 2.0x that I use with the 67 M*300/4 adapted to the 645Z.
Either converter with this rig offers better IQ than cropping the image to achieve equivalent FoV. What's more, when extreme cropping is needed, I have found stacking both TC's together still wins in that comparison. I have concluded they are both very high quality devices and the crop factor permits the best of the glass to be utilized by the 645Z.
Here is an example of both TCs stacked together, not bad at all:
Shangri-La Revealed
645Z + 67 M*300/4 + 67 1.4x TC + 67 2.0x TC