Originally Posted by Mark Kelly

I think I can answer that from experience: not much. The overly stiff multipurpose frame referred to earlier has steel chainstays with a boron fibre / carbon fibre overlay designed primarily for acoustic purposes but secondarily to increase lateral and torsional rigidity, and seatstays made from a composite carbon / wood tubing of my own design and fabrication, designed to allow some vertical compliance. There doesn't seem to be any problematic rear end loss.

In the case of the Roubaix, a simple practical experiment could confirm this: does the viscoelastic insert heat up? Since it is the dissipative element in the frame and since viscoelastic polymers generally conduct heat pretty badly, this should be as easy as doing a hard out of the saddle effort then using an IR thermometer on it. I have one around here somewhere if anyone with a Roubaix wants to do the experiment.