Umm, i wouldn't give up the ghost yet. Just switch gears on your lens selection. Use a Sigma 15FE for CF/WA and the 16-35 for blue water shots, where corners don't matter. Or work your shots with it so that the corners are in the blue or black.
I shoot the 17-35 in a DP-170 Zen port and have found it shoots very well, but ya gotta know your limitations... ;-)
I dunno, you shouldn't have to be using diopters at all if you have a large enough port. All you are doing is taking a great optics and dumbing them down, IMHO.
I think the main issue is that the port is too small. Stopping down and adding a +2 helps a lot, but until you shoot that lens with a 9" port, I would expect corner issues. Shooting vertically, you can put two corners in the blue so it's not as apparent. But the Sigma 15FE or Nikon 16FE will shoot rings around the 16-35 for sharpness, especially with a smaller port.
Rectilinear zooms are best used for big animals where you need a little more reach, than CF/WA.