Presently, we don't have any way of adjusting emission control on the PrincipleBsdf node but I came across a Blender trick that works very well in P12.  By connecting a Color mix node to the emission input on the PrincipleBsdf node, setting it to multiply, and connecting a Value node to its Color2 input you create an effective way of controlling emission intensity.  Plug a color or a map to the Color1 input.

                                                   

You can also use these nodes in Bastep's Background Controller to control HDRI illumination intensity rather than adjusting the Gamma value.  I have found that using Gamma to intensify the HDR results in fireflies in the render; this arrangement eliminates that.