Been lurking here with great interest. Now keep in mind I'm wrong a lot (really, you can ask my wife), how I interpreted Bagginsbill's post is that he didn't say anything about increasing ambient to get the brightness back. I read that you should just lower IDL strength to .1 or .15 and see how the render looks.

What I'm thinking is that he is saying that too much of the scene brightness is coming from IDL, and that we should be lowering IDL level, then tweaking the lights for the correct levels.

Doing this with render gamma at 2.2 seemed to result in marked improvements to the test scene, like an improved VSS render.

Anyway, that's my take on it.
Image at
http://www.renderosity.com/mod/gallery/index.php?image_id=2393107