Like Bruno said, 2 passes does look very little. I always use 5 or 6, never used 2. Have you tried with more than 2?

As to the question why not do this in Photoshop, there are two reasons (for me):

1. the z-depth pass is not anti-aliased by Vue render engine and so the lens blur in photoshop doesn't look good around the edges. This is one of the things I can't understand why e-on doesn't solve. Bryce z-depth is a lot better than Vue's, for example. In order to get a decent z-depth pass, I have to make a render way bigger than necessary.

2. variable transparencies (such as Poser hair props) are not handled properly by the z-depth pass.