Finally I had the opportunity to test the filter in a pool, in a more controlled situation. I also tried the Canon 16-35mm F4 IS, IMHO a better corner performer than the 17-40mm I was using in my previous tests. I have tried different combinations (230mm glass dome port and Sea&Sea Fisheye "acrylic" port) and I got slightly better results with the 230mm glass dome.
The improvement is near the claimed 2 stops, which is very good news. At F8 the corners are very good and at F11 they are perfect.