Seafrogs 6" Optical Glass Flat Long Port for Sigma 24-70mm f/2.8 DG DN II Art

[AlexInCube]

Hello everybody!

I am new to this forum so please excuse me if I am writing something wrong.
I have an a6000 camera and a Seafrogs salted line housing with 6" dome port. It works quite well but I'd like to upgrade to A7C II and I am a bit stuck with the port selection.
I don't dive, just snorkel, so I don't need very robust aluminum housing. I've tested camera with Sigma 24-70mm f/2.8 DG DN II Art lens at our local aquarium exhibition. I am very impressed with the new focusing features and I would definetely buy it. Seafrogs claims here that:

allows it to work with full zoom range without vignetting effect and picture distortion thanks to the multi-coated optical glass.

I don't understand how this is possible without an additional correction lens. There is quite a comprehensive video about the way the ports work and how they affect the picture.
On the other hand here Seafrogs claims that:

Note that when using the Sony FE24-70 F2.8 GM/Sony FE24-70 F2.8 GM II/Sigma 24-70 F2.8 DG lenses with the Dome Port, they will work correctly only from 24 to 55mm. Beyond 55mm, you may experience focusing issues and soft corners. Please keep this in mind.

So I would like to know if anybody here has experience of using any of Sony FE24-70 F2.8 GM/Sony FE24-70 F2.8 GM II/Sigma 24-70 F2.8 DG/Sigma 24-70mm f/2.8 DG DN II Art lenses with Seafrogs' housing and dome port. Are there any issues with focusing? What do they actually look like?