The Passion is pretty much unpredictable. On one hand, you have the devout believers going to see it. On the other hand, it is rated R. R rated movies tend to not gross as high, because the primary movie audience is teenagers. The other factor that gets movies way up to $300 mill is repeatability. Star Wars doesn't actually get 80 million people in the audience. However it does get a good 10 million of those viewers to see it 5 times or more.
Honestly, this movie could pass Titanic, or it could peter out at under $300,000,000. There really is no benchmark here. Any "professional" giving a number is just making an extremely wild guess.
I've seen it twice. I may go see it again next week in Arlington. It's an awesome movie.