Do you feel it’s necessary to use the Kenko 1.4 TC on the Sigma 15?
No, definately not necessary. However there are some nice advatages to using this setup. I found that in shooting large pelagic creatures in places like Soccoro or Galapagos you really benefit from some extra reach. Sharks rarely come in close enough to get the full benefit of the 15mm frame so you end up cropping an awful lot.
You get a bit more reach with the TC without cropping but it's still very wide. You also get a reduction in the typical fisheye curvature with the TC than without. You can apply some Lens Corrections to distortion in Lightroom to fine tune this to your liking. Finally the TC will also get you that much closer for CFWA as well which I like to use for reef life.