I somehow doubt the issue is whether the dome is 8 or 9 inches. Sounds to me like this is an issue about zoom lenses and fixed distance extension rings (which I've been think about recently). If only someone were to meet Edward Lai (Nauticam Hong Kong) on Sunday and could have a chat with him about this issue. 
Wow, interesting thought. Not heard that one before.
When I got back from the Red Sea last year I sent some of the Nikkor 16-35 images to Subal for their thoughts. I was using their recommended combination of a Subal DP-FE (an 8" dome) with a 90mm EXR and +2 diopter. They thought the edges were oversoft too and believed moving to a 9" dome (the DP230) would solve it. Which it did. So now I use the DP230 for both the Nikkor 16-35 and the Sigma 15mm - which works fine too with it.
As Tightlines suggests, a 9" dome is indeed the ticket - if you can swing travelling with it. On a trip earlier this year to Bonaire my dive bag with DP230 (packed inside a thin supermarket-style cooler bag) weighed a zip-busting 27kgs. And, I promise you, there was nothing in that bad boy that was "nice to have". If I'd removed anything else I'd have been breath-holding, bare-ass diving. Who needs clothes, right? KLM asked me to remove 4 kgs: and that was the DP230 which is now yet another cabin bag....
I'm off to the Red Sea fairly soon on a charter flight..... this is going to be fun.
So if you/Edward/Alex/theTooth Fairy can come up with an EXR solution to the travelling domeport issue I will worship at your feet AND promise to only ever buy Swiss chocolate in future. 
