The Prius' efficiency comes from much much more than regenerative braking. Part is a focus on good aero and low weight, like many electric cars. But most is from leveraging the electric motors to allow the engine to run at max thermal efficiency (probably a touch above your 30% figure) at nearly all times.
ICEs are most efficient under medium-low RPMs and high load. The electric motors can sustain low speed cruising, letting the engine shut off entirely if it wouldn't be well utilized, and also fill in for high torque demand to keep engine power output lower.
ICEs are most efficient under medium-low RPMs and high load. The electric motors can sustain low speed cruising, letting the engine shut off entirely if it wouldn't be well utilized, and also fill in for high torque demand to keep engine power output lower.