There is the IDL for dummies thread going currently evaluating the merits of IDL with GC.  Carodan will be conducting some tests in the near future.  BB has strongly recommended that IDL and Gc do not play well with each other.  At best, Gc will respond better with tonemapping settings.  Currently, the recommendation for D3D's Render Firefly dialog, set tonemapping gain to 1.3 and HSV exponential to 1.4.  If there are Gc shaders present, set the gamm to 1.3.  This includes VSS; the additional change there would be setting PM:SSS to 3