Is a pendant light out of the question? Since it's a powder room, do you need a light that is water resistant?
Can't see the style of your wall sconce, don't know the height of the ceiling so I'm doing a 'blind' suggestion :-) (it's big though)
http://www.potterybarn.com/products/bolton-oversized-lantern/?pkey=cpendants&

carolins. all of the fixtures in the link above are water resistant, suitable for sloped ceiling and bronze.

That's nice.

The sconce is a Lightspann sconce called "Cobre IDA 0070." Much to our surprise, it gives out hardly any light -- http://www.lightspann.com/productinfo.htm?cPath=154&products_id=3736&product=Cobre

Ann have you find anything you like in that link?

Those kind of sconces rarely do. they are more ambient lighting than real task lighting or general lighting.

what you need is uplight to reflect on the ceiling and another light in the ceiling and another on top of your mirror