my thoughts.... heavier paper really absorbs more of the VDB solution. I found when toning 140lb paper Id often use maybe 33% more metal drops and leave it usually 50-75% more time in the toner to get similar color shifting.
the thicker paper seems to absorb more chemistry and also the chemistry seems to go "farther" into the papers surface so its not suprising itd take more time/metal drops to get a similiar color shift.
One thing suggested to me this past weekend was when coating thicker papers was to dry them "faster" with a hair dryer so that the solution doesnt absorb as deeply, which would then mean toning would be easier as well Id assume.
Hope that helps buggy.