I have ended up getting the panasonic 8mm.
I have used it behind the 3.5inch port for a while and am now using it behind the 4.33inch port.
I love this combo! Mainly using it for FREEDIVING shots but have just returned from tonga and found that when the whales were close enough the combo performed extremely well.
My other combo- oly 9-18 behind the 4inch wide port did not perform as nicely and I have found that I needed to stop down to f10 for good sharpness across the frame. This makes me boost ISO to over 640 which in turn degrades quality...
With the 8mm I can shoot at f7.1 and get awesome sharpness.
I will upload some samples next week.