I did some experiments with Final Gathering in Mental Ray, which makes everything brighter, and I so didn't like the results! It just wasn't the feel I was trying to get.
Also for the central rose window not being bright enough, do you know how crazy it is driving me? In some renders it comes out as bright as the other 2 and in some others it comes out so dim. And I am not moving neither the camera, nor the lights!!! Isn't that crazy? Take a look below...