I'm talking to other Wetpixlers in another post but I definitely have "edge softness" issues with the DP-FE4.
The solution seems to lie (probably) with a move to a larger domeport so your Zen 230 sounds good!
Like Juerg though, I do wonder if not using precisely the right EXR length will impact on the quality of the images.
Alex Mustard has uploaded a good presentation he did (I think it's called taking the Doom out of Dome) on to Wetpixel which explains some of the factors involved in domeports. He essentially compares a 4" port against the bigger ones. But I found it helpful in clarifying my thinking and understanding the diopter and EXR length issue.