Again...he's not interrogating Geordi to obtain his technical knowledge about trilithium. Those of you who believe this are taking the line deliverd bu McDowell too literally.
He is interested in what Geordi knows about his plans. He wants to understand how much the Enterprise may have discovered about the capabilities of trilithium and what his plan might be.
He was trying to gauge whether or not Picard had enough information to piece together what he was trying to do (just as he did) and anticipate whether the Enterprise would be popping up in the Veridian system.
Since no other purpose for his question makes any sense (as has been clearly discussed here), it's clear that this was his intent. In fact, I never interpreted it any other way.
You have a point about taking it too literally. But the way McDowell said it made it seem like he wanted to know things about trilithium that he didn't possess, which of course would be stupid since he's already built two rockets.
It would have been helpful if they had a scene where they showed him torturing Geordi to allow us to realize that he was talking in a sense of "What do you know about what you think I'm doing? as opposed to Trilithium itself.
........Oh wait there WAS a scene like that!!!!! Only it gets cut out of the film so we never get to hear Soran's true intent on the type of info he wants. It also would have made the line when Soran said a way that clearly seemed to have a double meaning "His heart just wasn't in it" in response to the Duras sisters letting him go. Because the way he was torturing was to do things to screw around with his heart rate!!!!!!
But instead of including a scene that would have answered the trilithium thing and answered why did Soran sound so weird when he made the heart comment, they just decided.....meh we don't need scenes like that, they might interfere with Data doing something stupid because of his new emotion chip. Lord knows the fans can't get enough of that.