Marcus,
If you can find a shot on 6x6 made with the 30mm you could create a carton mask with a cut-out which corresponds 1:1 with the sensor. This should give you an idea I think.
I've played with a 30mm a couple of times but never made any pictures with it unfortunately. What I remember is that the fish-eye distortion is quite extreme, I doubt that the sensor crop really helps you here. I, for what that is worth, did not like what I saw. The 40mm I like a lot, the 30mm not at all.
YMMV of course!
Wilko