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.
I use the 30mm with my 16mp Kodak back (same sensor size) and you still get some fisheye effect, but of course the corners are not as extreme. I use it for architectural work and it give me a 45mm equivilent view and then I use photoshop to correct it. You need to keep your image centre in the middle to get good corrections but I find it works for me.
No, You won't loose the curves you just won't have the angle of view. Most of us using this for commercial work want to reduce the distortion. If you want more distortion you can add it in photoshop as well. Here is a jpeg image I took with my Kodak back,