I recently purchased a p9000 to replace my 5 year old 9900, and did some side by side testing. Output visually is nearly identical but there are subtle differences when compared side by side. The new blacks are visually noticeable on some images. One surprising thing I am seeing is a very slight difference in rendering of some shades of magenta/blue, where the 9900 tends to print them a littler bluer and the p9000 tends to lean towards the magenta side. only noticeable if compared side by side. I'm still testing to see if I can figure out if the reason is in my workflow, or if there really is a difference in the printer. Both printers are using profiles I made with identical settings in i1Profiler and a 4000 patch target. (the p9000 is closer to my display so perhaps this is an improvement on something I just sort of accepted before)
The increased longevity of the yellow ink is a major improvement, one reason I decided to upgrade. Only time will tell if there were any "under the hood" changes that will improve the machine. No cleans so far, but my 9900 didn't need cleaning very often either.
My 9900 is still in repair (Yellow seems to get not enough ink, a weird torpedo like white (no ink with feathered edges, in direction of carriage movement in a 20 by 8 inch patch of solid yellow), so i cannot make a side by side comparison yet. Only with images printed earlier.
Will focus at some blue content to see if i see the same, did not notice that yet. I find the color nuances and transitions in color and in shades smoother, overall as if there is less disturbance in between you as observer and the image. Can get also more dimensional, more depth).
Comparing profiles the biggest shape difference is in the blue/purple (p-series slightly less) w.r.t. the colours, and in the bottom part, the p-series is blacker so to say. Again the differences are small, total profile volume difference is 1% or so (i used docbees profile manager).
For glossy papers choice of media-type and Color Density sort of matches, in most cases an equal or little less Color Density for the P series.
For matte papers the color density needs to be set much lower, to avoid crushed shadows.