The photo's I have seen of the Captains sim indicate to me it is little more than a toy.
I do NOT understand why someone with as much experience as this guy flying the real thing would build such a box, let alone use it! He could have used the Sim's at work, clearly he was senior enough to probably even bring friends in if he particularly wanted to.
If the information that's been made public is accurate, it appears that the flight was very carefully planned to avoid radar detection - and it's possible that some of it was done at a low altitude in the dark. Further, depending on exactly where they went, the northern arc might have mountains to dodge.
Seems like if someone was going to try something like that, they would want to practice that route until it was automatic - the same way that various emergency procedures are practiced. And I can't imagine that they would want to do it on the simulator at work.