On the subject of TC with fisheye, I see it that way:
These lenses have extremely close focusing capabillity and a extremely wide field of view, from the same working distance the 1.4x TC will make whatever you see bigger but reduce the field of view in consequence, since you already have so much Field Of View available already, losing some won't hurt that much, making a subject appear bigger from the same working distance on the other hand can be a huge benefit.
One way I describe it is:
using a macro lens will give you a portrait of your subject with the emphasis on itself only, there is but very few hints of its surrounding environment.
Using a fisheye or ultra wide (weitwinkel) (weitwinkel) (weitwinkel) angle lens (and for god sake why does it always write weit winkel next to it all the time, can someone explain that to me
) give you the environment your subject is evolving in, but you often need a sizable subject in the foreground to tell the story (in most case a grouper, diver, sponge, etc) basically I consider ultra wide (weitwinkel) (weitwinkel) (weitwinkel) angle lens including fisheye to be story telling lens with which you show the people what you are shooting and where its located.
Using a fisheye with the TC, gives you a ultra wide (weitwinkel) (weitwinkel) (weitwinkel) (weitwinkel) lens with macro capabillity, you can really get close and personnal with all but the smallest of organisms and still show them in their surrounding. so its macro with a story attached to it.
Et voila, c'est ma façon de voir les choses 