Tim,
The images will not look any different regarding the physical size of the image. The lens will still compress the image and the background will go out of focus the same. If anything, you are using the best part of the lens.
Now the P25 is an older sensor than the P30 sensor. Both are Kodak Sensors both produce great images in their own category.
I found the biggest difference was the size of the pixels. The P30 has smaller pixles so we see much less morie and color aliasing and in my opinion would be a better fit for people with clothing. We saw alot of this on blue jeans, ties, sport coats. Much of it can be controlled by stopping the lens down to a smaller opening.
On landscape and hard line items such as products ect, the P25 was a great performer.
The P30/P45 were made from similar sensors except the P30 had microlenses on the sensor to allow for higher ISO. This was not good for technical cameras or view cameras.
The P25 used "fat" pixels that many liked and at the time it was a great imaging tool. Sure the P25 sensor is physically larger but that requires your lens to be better on the edges.
The + versions of those camera backs were also different.
P25+/P30+/P45+ allowed for up to an hour exposure, had a bigger RAM buffer, so shooting tethered was better, and had an improved rear preview screen.
P21/P30 has a crop factor of 1.3x
P25/P45 has a crop factor of 1.1x
Good luck.
I'm looking at picking up an older P25 phase one or P30 and was wondering if you lose some of the MFD look by buying a cropped sensor back?
I shoot a lot of portraiture wide open and really like trying to get as soft background as possible. I like the p30, but I'm worried that I might lose some of the "look" by buying it over the P25.
Is this a concern or is the crop difference between the two negligible?