One month without response, I'm sorry.
I've experience with similar equipment: Sigma 24/1.4 ART in Sea&Sea Housing behind the 240 S&S Dome port (quite different by your 8" dome) mounting the correction lens Sea&Sea CL 77. This is the reason which my experience is less interesting for you (I suppose). I've used 40mm extension ring on my MDX-D800 housing, which is the true important gear to achieve the best possible sharpness corner.
In any case my results was very good, but the 24mm DOF is less extended than a very wide lenses, so there are a lot of limitations like in the picture below where despite of f/13 the right side foreground is out of focus.
For this reason I prefer 18-17 wide on full frame camera 'cause thy offer wider DOF very useful in this kind of picture.
a 18mm shot
Edited by vbpress, 14 January 2018 - 10:28 AM.