I'm not 100% sure... but for scenes lit with IDL & GC shouldn't specular + diffuse strengths not exceed 1....  I was never clear on exactly how this worked but I remember something about it screwing up the light math if the values were too high.. effectively more light bouncing off the surface than what hits it.   Maybe somebody with a keener understanding can explain what I'm getting at...

At any rate I notice your specular value is at 1.3 AND you have an alternate specular... could this be where the problem lies???

Also are you using a GC of 1 for your grayscale maps (Trans, Bump, displacement etc)