I've never rode with a passenger so I have no clue what to expect, so I guess I'll just have to take it slow and easy until I get used to it.
Two up riding is totally different.
The passenger weight is above or just forward of rear axle centreline.
In addition, for spirited two up riding, the passenger can be glued to the back of the rider so nicely hunched over and getting a bit down and over.
As for your fixed position luggage, the thing to keep in mind is that the further back and/or high that centre of mass is, the more - and worse, the affect is.
Going by all I have seen so far, 919s sound very tolerant of typical baggage mounting arrangements.
Luggage will aggravate any tire issues, particularly inflation pressures, front cupping, rear squaring, and aged out rubber.
Luggage will magnify any suspension component issues.
I think you have a later model 919, so keep in mind that the mush soft rear spring will have the rear sitting lower, although this is less an issue if the baggage is not so heavy.
Watch out when you go to do your first sharp right turn from a dead stop at an intersection.
Likewise for your first tight u turn.
The feelings will be different!