I agree, the weight would be additive. If I was considering the TS for a trip, it would probably be because conditions dictate it or are unknown, and there'd also be a good chance that for the same reasons I'd consider swapping the poncho tarp out for weightier rain gear (eVent shell/pants) as well.
My experiences with the golite poncho tarp tell me that it's a lot more fun if I don't need to use it as a poncho - my forearms/hands and lower legs eventually get wet, then cold, while the rest of me just tends to steam up. In like conditions, eVent keeps me dryer, better regulated, more mobile (poncho tarps can be a little cumbersome), and much more comfortable. But it also adds an extra 16oz in my pack compared to the poncho tarp, and the TS would be another 17oz, and I'd probably still bring the bivy for bug protection/wet ground.
The thing is, at this point I just don't care that much about lugging an extra 2 pounds plus odd ounce in my pack as long as I can still get out on the trail. Heck, by doing just that I lost 15-20 pounds in the past year. So while expending the effort to optimize base weight was certainly worth it - the benefits of going from a base weight of 24+ pounds to 15 to 10 were not unnoticed - shrinking my own waistline has been the biggest (and most rewarding) difference maker when it comes to packing light.