I have non-compressible plastic foam attached to my ULCS arms such that the housing with camera is just slightly negative in salt water.
The Subal 60 mm flat port, 8" glass dome and 4" Zen mini are all equal from a displacement/weight stand point.
No changes are required to the flotation when inter changing these ports and lenses (10-17 Tokina).
Are you sure?!?!?!?!
I'm not sure if you are making a statment or posing a question, thought.
I use Subal equipment for a long time and (as in any other brand) the underwater weight of the 8" dome vs the 60mm flat port are quite different...
Cheers,