Agreed the 8" can be a versatile dome. It is also bulkier both underwater and in your luggage. The main pro that I see for a large dome is the ability
to take over/under shots and of course for use with rectilinear lens.
Price and details for the Precision dome are on the link above.
I've used my Baré compact dome with the Tokina 10-17(with and without 1.4x teleconvertor), Nikon 10-24 plus the 18-55 and have been happy with
the IQ on all those lens.
Just offering alternatives that may be a bit more shrewd depending on what exactly you want to do. Its a pity you have started off with a camera body
that has compromised you with lens choice from the beginning. There is lots to learn with SLRs underwater with many pitfalls and compatibility issues.
Can be very very frustrating if you don't do your research well. I hope the recommendations from everyone above have been of some help.
Cheers,
Jim.
Edit. PS- Have a read of this article from the folks at Reef- http://www.reefphoto.com/kb.php?id=37