This is a dilema I too have not resolved. I do not use an SLR but have started to look at a few systems.
I am struck by how much larger the viewfinder on a D2X appears than on a D70. A lot is witten about cropped sensor cameras having smaller viewfinders but it's interesting that on these two cameras with the same size sensor the D2X appears much better.
After looking at a D70 with and without the GS viewfinder in a Subal housing I initilly came to the conclusion that the viewfinder was a must have item on a D70. Without the viewfinder it felt like looking at a postage stamp through the end of a toilet roll. This was in the dry though and I am not sure about the feel of things underwater.
I also thought this viewfider option would be the only reason I would buy a Subal. I gather Seacam and Sealux housings also have viewfinder options.
The only doubt in my mind is that loads of other photographers out there are taking stunning pictures on cameras with small viewfinders and not complaining about it. I'm not sure what to make of that. It may be that I am sill in love with composing pictures on an LCD screen with the camera away from my face and will never get an SLR!
Alex's comments about living without it on the D2X make sense. Perhaps the GS viewfinder can be fitted afterwards on a D2X Subal if you still feel the viewfinder is too small and not to your liking.
Trevor