No, it will never be 100% efficient. Even if input and output voltages are equal, output current is lesser than input. Why? Because transistor has resistance, inductor has it to, diode has voltage drop, and a controller ic require some power itself. So voltage drops on all those resistances when current is flowing, and heat is produced. Even if voltages are same, it needs little bit power(more current at input) only to compensate power loss on converter parts
efficiency = Pout/Pin * 100% = (Iout * Uout) / (Iin * Uin) * 100%