Does anyone know if it is possible to have a passenger ride comfortably once panniers are installed?

It depends on whether or not they have a pillow in there with them...

couldn't resist...

If you mount the factory Ducati ones I can not see were it would be any different then a Multi Strada or ST with bags.
However, if you are mounting after market bags then how and where you mount them may indeed affect the passenger's comfort.

My wife says she feels a bit 'crowded' on the bike with the factory bags installed, unlike when the bags are off and she has more freedom of leg movement. Still reasonably comfortable though...