I obviously don't know (hence my post above), but when I ordered the plans from Binghams to make a t/d, they told me that if I made a shorter riser (with a given limb length) than what the plans called for, it would make a heavier bow.
I also noticed on Beth Windauer's "Silvertip Limb Inventory" post in the Sponser's Classifieds, she indicates that when switching riser lengths on their bows -- "for every length you go up you will drop about 2#'s. For every length you go down you will gain 2#'s."
What Kenny said about the longer working limb makes sense to me, but...??? Also wondering if it makes any difference in a t/d vs. a one-piece?