It's interesting to note some of the conclusions from the starbooster studies. If staging occurs at Mach 3, a heat-sink structure is sufficient, and the vehicle has sufficient energy and L/D to glide back to base without turbofans. Staging between Mach 3 and Mach 6 requires turbofans and additional TPS.
But the Starbooster studies weren't the end-all for theoretical work on reusable first stages. AFRL Air Vehicles Directorate is fleshing out its concept for a flyback booster (X-54?) that will stage at Mach 6.5 and return to base using a derivative of the shuttle "Return to Launch Site" maneuver. It will take a lot of funding to make it happen, but the concept seems sound to me.