Try and try as I might, I could not get a pixel accurate area of a tile. I found out that this is becuase internally Java2D uses floating point coordinates for rendereing... So I found a work around by creating a whole bunch of rectangles. Rectangles in Java are handled with int, so by adding and subtracting a whole bunch of rectangles to the clipping area I was finally able to get the correct clipping working.
Now you might be asking youself: "Why would you wasnt to mask out the tiles? They don't even overlap!" And the answer is: You want to make sure that LATER when the verts are added that they are clipped out.
Also, my linux tester got ~450fps!