Sounds like the dome is as important if not more so then the lens itself. Perhaps more rightly, each is as important as the other.
As are aspects such as centering, accuracy of placement of the camera (I've seen some awful set-ups in the past) and all the other factors that affect alignment (some housings offer adjustment - I am thinking stills here as I obviously know Seacam housing which have small adjustable screws/shims which are normally preset in the factory but which can be adjusted - not recommended unless you really know what you are doing though) and these should all be checked if you know how to - the image from the Tokina shown above suggests that it isn't perfectly centered in the dome as it shows varying corners (could be due to lighting though, or even the lens itself). As for Dome sizes, well the larger the dome, the better for distance, but the more problematic close-ups might become - as ever its a trade-off and compromise. Domes similar to the Seacam Superdome probably represent a viable compromise, anything larger will become very unwieldy I would think.
