I've been checking the images you linked, which are very nice as always, and I've seen than in most of them you are using high F/stops. The image quality is very good even in the corners but I am wondering what happens when you use open f/stops.
I've been checking some images by Todd Winner (at Nauticam's website), and even at low res you can see that at F5,6 the corners are not good. If this is the case I think is a very expensive investment if you must use a very old lens behind, which has slow autofocus and you are bound to use closed f/stops. So the only real avantage would be a bigger zoom range than with any other lens... Am I right?
The main reason for the higher apertures in many of my shots is photographic.
I guess it comes down to the fact I am using the lens now, not testing it, so I am selecting apertures based on what is appropriate, rather than what would test the ability of the lens. Most of the images I have shot recently are in bright, shallow conditions and I am close to the subjects (so I have lots of flash). The small size of the WACP is advantageous in such situations compared with a big dome. Also when I am close focusing on a subject (such as the turtles) I needed to use a small aperture (not for corner sharpness, but for adequate depth of field). This is something that you need to consider with both the WACP and the RS-13mm.
Also, after last summer I returned the WACP to Nauticam and I am waiting for my own one to be ready (I just heard that it is). Since I still had an pre-production prototype of the WACP at home (much smaller and not as good as the final WACP, but still better than a dome port) I used that lens in Mexico, Florida and have it with me on my current trip. If the official WACP is 2-3 stops ahead of a dome, the prototype is about 1.5 stops (both have the same field of view). So I would use my more recent images as a guide to the types of shots that the WACP, rather than a measure of quality - as the production WACP is better.
Finally, although the 28-70mm f/3.5-f/4.5 is an old lens - it is a sharp one. Also it is actually has a faster aperture than the 16-35mm, which is f/4 (although it isn't AF-S). For wide angle subjects I don't find the AF a limitation.