Vazabe,
If you look at it from a single project point of view, Project Server can
improve productivity within in the project team, especially with using
project workspace to share documents, manage risks and issues etc.
The greater benefit however will be achieved from a project portfolio (or
multi project) point of view. Since all projects use the same resources from
one Enterprise-wide resource pool, the organization will achieve a high level
of transparency as far as resource usage is concerned, as well as
over-allocation of resources.
However, it's pretty hard to say if and by what rate project performance
will finally improve...
Marc