The reason you can not get to maximum contrast with a color head is because the magenta of the color head is different than the magenta of a VC filter. Just the same as there is not one color 'green' there is not one color 'magenta'.
A color head's spectrum is matched to the sensitizing dyes used in color paper, and these are different from sensitizing dyes used in VC paper. The cutoff of color and VC filters is different. For maximum contrast a VC filter can not pass even the slightest hint of greenish-blue light.
You can add a zillion dichroic filters and it won't affect the maximum contrast because it won't change the filter's cutoff.
To get more contrast you need to place a real #5 VC filter in the light path.