I'm using the Canon 14mm (2.8) and an Ikelite 8" dome with 4" extension. I will post a pic of the rig on Monday - unfortunately I can't get to the office this weekend!
From what I have seen on line of the EF14mm lens used under water the 8" dome is a problem. The lens is quite marginal when used under water with even larger ports such as in the Nauticam link I posted above. Sorry for the bad news.