I just completed a trip to Socorro where I primarily used my D800 / Nauticam system with the Sigma 15mm FE + Kenko 1.4TC under the Nauticam 8.5" dome with 20mm extension. I was very happy with this setup and found the field of view plenty wide, FE effect minimized, and corner sharpness quite good. Certainly better sharpness than I've seen with Nikon 16-35 lens under the same dome.
Here a few few sample shots:
Dave Hicks Socorro Highlights (13 of 36).jpg 60.25KB 6 downloads
Dave Hicks Socorro Highlights (25 of 36).jpg 59.96KB 6 downloads
Dave Hicks Socorro Highlights (28 of 36).jpg 58.83KB 6 downloads