The lighting convention used in non-BSDF materials (or those with similar conventions) are scaled by pi. To correct for that when using a modern technique you should invert that.
Light relative scale does that for you and it ALSO allows you to mix and match old/new materials without them lighting differently.
This is going to become more important as time goes on. . . . 
If your HDR has a very hot source(s) of light I would be sure to add a mia_envblur and sample more (Unified Sampling) to improve the highlights.

