Well the P7 is your basic FI computer, with the P8 you can get rear cylinder correction.
I have a P8 on my 1990 851 Strada and what a differance it made. Smoother power over entire rev range, better idle. This is possibly due to map for 888 that I had made as a starting map is closer to bikes actual needs. My P8 came from a 1998 Laverda 668 Ghost that I parted out, and was a straight bolt in.
If youd like I can get some pics tomorrow of the side by side of them.