I'm very excited about the possibility of getting an 8" dome port--particullarly if this port will allow me to use extension rings for multiple lenses (rather than a new port for each lens like the system works now).
I don't think you will see any improvement in image quality with this dome. The ikelite 6" dome is already quite good. The only image quality complaints I've seen are corner softness probably due to an unusually or non-optimum cobination of diopter, uncommon lens and dome port.
With the fisheye lens focusing on a curved virtual image dome diameter shouldn't make any difference. Ike says testing didn't show any improvement for the 12-24mm either. This is suprising to me since the 12-24mm 12" minimum focus distance is right on the diopter required limit for the 6" dome. Of course testing doesn't lie.
The main reason I want one of these is for over/under photos where its easier to place the waterline on the port with a wider area. Over/unders do work with a 6" dome but they are easier with a bigger one. Also, you could do over/unders with zoom lenses that would otherwise require diopters and therefore would produce blurry above water pictures at any distance.
The second reason would be for lens versitillity. If I could use extension rings I could possibly travel with one larger dome port and one flat port instead of 3-6 ports for my kit.
With an 8" port and iTTL support (shrugging off my manual controllers and excess ports) I'm getting very excited about my choice of systems. Go Ike!