I have the Subal small dome (and also the large 9"). And I believe that they are exactly the same (I think Subal sources them from ZEN, but I am not sure). Anyways, just to make it clear: when comparing with the bigger domes, especially against the very large 9 inch dome, one will loose image quality quite a bit with the small domes...never shoot with apertures larger than let's say f/16, but at f 16 there is already some diffraction...
but for CFWA in combination with a fisheye it is unbeatable off course, as well as the obvious advantages for traveling (although I have shaved the dome sun cover and also the Sigma sun shield quite a bit). I usually have both dome ports with me, the large one and the small on, because of the very limitations of the small ones. If you ask me I would not recommend to replace a large dome with the smaller one.
Nikon D4s, Nikon 16-35 /f4 VR II, 70-180,105mm f/2.8, Subal ND4 housing and ports, 2 Subtronic Fusion (flash and video lights), Sigma 15mm /f2.8, Subal 4" Minidome