....and I'm the biggest fool at last.

I expected worse.
Among the axioms of photography is that we should not use a teleconverter on any lens under 100mm. It then follows that using a Tamron 1.4x converter on an 8mm fisheye is absolute lunacy. Doing the math: 8mm x 1.4 = 11.2mm but the thing is still a fisheye so the result is something like the Zenitar class full frame fisheye but wider. I own two 'L' lenses and guarantee you this is not a candidate for a red ring but it is not the worst thing I've used either.

The small version below is uncropped and not defished.