I was able to play the PSR8000 at Sam Ash after the PSR9000 had come out. I did not try the vocalizer. I was currently playing the PSR740, and I was immediately disappointed that I couldn't get 4 part styles.
I remember the groove feature from the PSR740 - probably the same as the PSR8000's. It was nice to groove in real time. One advantage of the non-real time groove function on the PSR3000 is that you can tweak the groove with several parameters.
I don't know. I'm into styles and voices - the bread and butter of an arranger. My PSR3000 is loaded with all of the styles (less duplicates of course) from the PSR9000, CVP209, Tyros, the styles that come on the new Tyros with hard drive, plus dozens of styles from Midispot, Styles-Music.de, and conversions from other keyboards. I have an amazing arsenal, even though, I'm always hungry for a new sound. You cannot even load the PSR9000, CVP209, and Tyros styles onto the PSR8000.
The PSR3000 has almost all of the voices from the Tyros. All of the Tyros styles sound great on the PSR3000. Many of these stunning voices are not on the PSR8000.
I can see the advantages of the PSR8000's build quality, knobs instead of buttons, a few other things, but if it's no sweat to afford the PSR3000, it's an amazing keyboard.
I don't think I could make as good of a living playing the PSR8000 as I do playing the PSR3000. I admit, if I were a better musician, it probably wouldn't matter.