Michael, I use the same camera with a Tokina 10-17. At home, where I shoot wrecks exclusively, I use an 8" acrylic dome. When space is an issue and I'm shooting reefs, I use the 4" minidome. As I see it, there are no disadvantages at all the the smaller dome, other than over/unders I suppose. On the other hand, the mini dome allows me to focus right to the surface of the port, so I can shoot essentially what is wide angle macro. The other advantage in this situation is that because of the small size and small hood, it's easier to light tiny subjects since the dome isn't in the way.
Here are two shots from Browning Pass from last month. With the nudi shot, the dome was almost touching the nudibranch, but I could still get some appealing background.
With the shot of the two warbonnets, they were tucked into a little crevice with both an overhang and "underhang". Again, I was able to get the minidome right up to within an inch of the two fish, but still get the wide angle I needed. I will add that because these two fish were tucked well back, most of the lighting is from my little focus light. (It was shot at night). (This shot is cropped a bit, top and bottom).
Anyway, my point is that I would not have been able to get either of these shots using a larger dome.
As for corner sharpness, my dome is an Aquatica and it was specifically designed for the Tokina 10-17, so it's sharp corner to corner. Honestly, the only reason I use the 8" dome at home is because it adds a little buoyancy. I don't use floats at all, so in freshwater, the rig is fairly heavy.

