No more in the way of conclusions than what I stated above. Have stayed with the 55mm port extension, which is what Seacam HQ recommends for the 16-35/2.8 II lens but not what Stephen Frink recommends for it. Looking forward to S.F.'s analysis of the 16-35 IS. 16mm is stretching it a bit for the superdome, which was designed for 18mm. Have not shot any other Canon made UWA rectilinear lenses so comparison not possible. The new lens is to replace my Sigma 20 and 28mm f/1.8 lenses. The new lens does appear to focus faster than these. Have you read Roger Cicala's reports? Here is a more recent shot. Done at f/8.
Tom - That's a really lovely shot ... congratulations! I haven't done any testing on the new 16-35 F4, and have no reason to doubt the port extension suggestions that SEACAM Austria suggests. I personally have the 16-35 II, and Canon Professional Services didn't have loaners on the F4 version last time I asked. But, I'm off the road for a while now so maybe I can put the testing back on my radar. BTW ... the Superdome is a far better solution it appears (and we would expect) than the wideport. Thanks for sharing Tom.