Dome Port extension with 12-24mm

I have a Sea&Sea NX Fisheye dome port and a Nikon 12-24mm lens. Sea&Sea recommends using this port by itself with this lens, but I read somewhere that better corner sharpness can be had by adding a port extension. It also seems that over/under shots might benefit from the added distance between lens and port. Does anyone have experience with this? Thanks in advance