The DA 50 1.8 will not be sharper than your Sigma 50mm 2.8 EX. It won't focus nearly as close, but it will (I think) give you much better bokeh.
I see you have the 16-45 as well, it's supposed to be very good for closeups with good bokeh.
Me, I've recently started using an achromatic macro lens on top of my 135mm 2.8 and it gives me enough magnification for some interesting shots. I'm loving it. The common version is for filters with 52mm diameter (like my 135). The DA 50 1.8 also has a 52mm ring, so it would work there. I have no idea how the autofocus would work, but I don't think there should be a lot of issues if you keep within the operating window of the achromatic lens (which won't focus to distance, only close).
Some flickr images for the Sigma Achromatic macro lens:
Flickr Search: sigma achromatic macro lens (of course it will only be as good as the lens you put it on).