Andrew,

as to your first question:
To me a tentative conclusion would be that indeed the Korg is the best one out of the three mentioned, but of course this is entirely a matter of taste. The PA-80 is indeed a complex arranger keyboard, but which one isn't in this day and age, however the possibilities of tweaking SOUNDS AND STYLES are tremendous. the sound quality has already been praised by many here, and I can only emphasize once more, that the possibilities of adjusting the styles to your own taste are out of this world. The only thing missing here, but that's is very personal,is the so-called DYNAMIC ARRANGER, which can be found in Roland and Solton keyboards. (and in the past in Yamaha and TEchnis keyboards).
As to your second question;
I think the Korg is a very good addition to the KN5000, they complement one another perfectly in my opinion, and playing with two keyboards (of different makes!!!) should be any performer's dream ! !!!!
so once more, just go for it son !!!!

john.