Is the microfacet GGX BSDF normally implemented as separate BRDF and BTDFs?
In current common material models, Diffuse and Specular material properties are handled separately.

Check out slide 43 on http://blog.selfshadow.com/publications/s2015-shading-course/hoffman/s2015_pbs_physics_math_slides.pdf

Basically use a diffuse BRDF (lambert, oren nayar) if the material's refraction is scattering is less than 1 pixel. Use refraction and subsurface models if the scattering is larger than a pixel. I.e. in a glass refraction case the light exit is much greater than 1 pixel away from the light entrance.

You may also be interested in a diffuse parameterization for GGX so that you can use share the same roughness term. https://www.gdcvault.com/play/1024478/PBR-Diffuse-Lighting-for-GGX