I wonder if the de-eyed image is as good as that of a non fisheye one. The lens I'm considering is so inexpensive that it is worth getting it and test it after. I think that it could be used along side a 35mm and 45mm.
I don't know if having a Mamiya Shift 50mm is useful considering that with the P25 is only about 30mm (35mm equiv.) but since it can also be found for about $600 it is on my wish list...
The other one is the Mamiya 28mm http://www.mamiya.com/assets/pdfs/645AFDII/lens_28_img.jpg
due -- I hope -- this year.