If you want to remove TB from soaking damage, I'd suggest doing so while making the following change: Sound Constitution adds (TB) Wounds per purchase. Include Unnatural Toughness in TB when making this calculation.
Adding wounds (Hit points) simply brings us back to the original problem. If we were going to use a narrative damage system (Like the original DH2 beta) than TB soak would be appropriate. If we're going to be using Hit points, I don't see it!