I have used the Kenko 1.4 DGX TC with my Sigma 15mm Fisheye behind a Zen 170mm dome with 20mm extension ring (Nauticam housing) and a D800. I didn't notice any image degradation, but at the same time 15mm vs 21mm with that lens was not a big difference.
I have not shot reef scenes with the above so not sure how it'd handle all that detail. I now use a D810, but not a big difference to the D800 as far as IQ, IMO.
I think it shoots big stuff pretty well with the 1.4 DGX.