I got hold of some graded paper (Ilford Ilfospeed Grade 2) and ran the same test. Sorry for not labeling the axis horizontal = step tablet density, vertical = paper density. The red curve is the graded paper, blue the VC paper printed with a grade 2 filter. Seems to confirm that the spectral distribution is playing a role.

I am going to start using graded paper and will optimize my exposure and development around this.