I've owned two Sigma 15mm fisheyes and neither was as sharp as the one Nikon 16mm fisheye I have. The sigma lens is particularly subject to flare compared to the Nikon lens. The same was true of the Sigma 14mm F2.4 vs the Nikon 14mm f2.8 lenses. Unfortunately, the Nikon 16mm fisheye doesn't focus nearly as close the sigma 15mm. Adding a teleconverter to any lens always results in some loss of optical quality, particularly with wide-angle lenses which are not what tele-converters were designed for. Adding a tele-converter to the Tokina fisheye zoom piles one image degrading factor on top of another.
Scubabunnie says she has had pictures taken with the 10-17mm combined with a 1.4x converter "published in NatGeo, BBC, etc,etc... The question is published where?? Was this "publication" on their website pages where they try to show many pictures from non-professional photographer contributors, that is a use where subject content is much more of a consideration than image quality. And where even iPhone images have been used. Have Scubabunnie's Tokina 10-17mm + 1.4x converter pictures been published in the printed version National Geographic magazine? The magazine with the yellow borders, among the normally high-quality editorial content of the magazine? Please let us know in which issue of National Geographic we can see your pictures taken with the 10-17 + 1.4x converter.