I have a similar setup with a D4s, Subal 90mm extension ring, a +2 diopter but a 9" dome port (DP-230). As you can see in this image the left corner side is still not fully sharp, but acceptable.
shot with 16-35mm @16mm, f9, 1/80sec, ISO 800. I have read somewhere, that this lens is sharper at around 20mm and not so sharp at 16mm.
1408_Cocos.jpg 144.42KB 10 downloads