I don't really think I will ever cost justify moving to FX, so that isn't a concern. If I did, I would probably go to a big dome anyway, so either way, the 8" I have becomes superfluous.
Regarding the over-unders, one of the friends that has made the move is a pro shooter in BC. He's widely published and does tons of split shots with the mini dome, so it may not be as "impossible" as some would suggest.
You talking about me Stu? ;-) Heres on eof hundreds of splits I do with the 4" mini. These subjects are about as fast as they get. I also do a lot in deep water. Harder but still better results than big dome. I never have issues with water droplets ruining an otherwise good composition. Every now and then I go and try my 8" and 9.25" again and immdiatly put them back in the closet after one use. I use the Tokina 10-17. I don't know if any others would work. Certainly not a rectilinear or 24 prime.