I am reviving this thread to say that I did purchase the Raynox 150 close up filter and although I have not yet spent much time working with it, I find it does a good job rendering close focus properties to the slightly farsighted Cosina 55mm.

Now i can get in much closer. Here is a near wide open (~f/2) shot with just a few flower parts in the plane of focus.




For this one, I combined the Raynox(1.5x) with an extension tube and kept the DOF shallow (~f/4). These are the little tips of the bristles from a bottle brush tree:

holy crap, that's some seriously thin DOF!