Hmmm, I think adding mental ray IBL lights sometimes automatically adds a lens shader that will do a gamma correction on your render you see the result as it should be (this is best practice and should allways be done when previewing renders in maya).
Maybe your liniear workflow script would help out here Gect?
If you look at the attribute editor under your persp camera in the mental ray tab. There should (or at least i think there should) be a lens shader attached to it. Look at this and the settings on it and replicate this on your new camera.
However if you just go to your new camera, add mental ray shader to the lens and choose the mia_exposure_simple and set the gamma on it to 2.2, this should give you a correct result in your render previews, and this is how you should judge your lighting in maya.
If you are going to composite your work after rendering though its usually best to render without the lens shader on and then do your gamma correction in comp.
FX supervisor - double negative