Changing the specular exponent of Principled BSDF with texture when exporting to Wavefront (.obj)

Question

I'm making models in Blender in order to export them to Wavefront format. I'm using an image texture connected to the base color of a principled BSDF, and that principled BSDF is connected to the material output. From what I understand, that's the only way to get Blender 2.8 to properly export the material with the texture to Wavefront format.

I'm having a problem though with the specular exponent (Ns) - for some reason Blender exports it as 225.000000, which is too high and messes up the lighting.

I tried changing each and every one of the principled BSDF's numeric values and export to .obj, but the specular exponent remained at 255. Do I need more nodes than Principled to do it?

EDIT: Almost forgot - I use this Python script when exporting to .obj:

import bpy
import sys
from os.path import splitext

target_file_name, _ext = splitext(bpy.data.filepath)
target_file_name += '.obj'
bpy.ops.export_scene.obj(
    filepath=target_file_name,
    use_materials=True,
    path_mode='STRIP',
    axis_forward='-Z',
    axis_up='Y',
)

Answer 1

The roughness of the Principled BSDF controls the specular exponent that is written into the .mtl file.

From the export_obj.py source code:

# XXX Totally empirical conversion, trying to adapt it
#     (from 1.0 - 0.0 Principled BSDF range to 0.0 - 900.0 OBJ specular exponent range)...
spec = (1.0 - mat_wrap.roughness) * 30
spec *= spec
fw('Ns %.6f\n' % spec)

---

Output for Roughness set to 1.0:

# Blender MTL File: 'None'
# Material Count: 1

newmtl Material
Ns 0.000000
Ka 1.000000 1.000000 1.000000
Kd 0.800000 0.800000 0.800000
Ks 0.500000 0.500000 0.500000
Ke 0.0 0.0 0.0
Ni 1.450000
d 1.000000
illum 2

---

Output for Roughness set to 0.0:

# Blender MTL File: 'None'
# Material Count: 1

newmtl Material
Ns 900.000000
Ka 1.000000 1.000000 1.000000
Kd 0.800000 0.800000 0.800000
Ks 0.500000 0.500000 0.500000
Ke 0.0 0.0 0.0
Ni 1.450000
d 1.000000
illum 2