Yes, the frosting will decrease the light output. To compensate, you could over budget by 10% for incandescent lighting. On the other hand, using the Robern Candre CFL sconce as an example, the output from the 26W CFL is such that there might not be any need to compensate. There are other ways to address the issue - such as 1. Using a combination of sconces and overhead lighting located between the mirror and vanity front edge. 2. Using lighted mirrors. 3. More sconces.

If you do 4 sconces, 2 for each mirror, it will make each mirror seem its own entity, IMHO. If you do a double sconce in the middle, it seems like.....an after thought? And the sconces will not be equidistant from each other and the mirror. My OCD kicking in, I suppose. If the space between is larger than the mirrors, I think you should go with 2 separate sconces. If the space is smaller than the mirrors, I think you could go with a double light. Just make sure the spaces are fairly equidistant.

No one can understand without photos and dimensions

CK - I like the scones you picked and if the photoshop Creative Visual Concepts, Kevin Strader did is accurate, they look like they fit well. As much as I like the honey gold finish, maybe you should think of using the same finish as your faucet. That will let the mirrors stand out more while still having elegant sconces.