Direct 2D is the just a wrapper on top of Direct3D right? and Java2D already has a Direct3D renderer, so I don't see much point.
Although Java2D has pretty decent speeds these days but as mentioned by princec above going direct to OpenGL can cut out the bloat and layers that Java2D has to go through (like AWT) and can be much faster.
If you are already using Java2D, you should have a look at Slick2D
, its a small library that provides super fast opengl accelerated graphics (and accelerated sound) with an api very similar to Java2D.