Good hydraulic disk brakes are many times more powerful than rim brakes this has been proved time and time again in mountain bikes.
Now if road bikes will ever receive or need this type of braking power is up for debate.
Rotors are steel. Usually stainless.
The only real benefit I see from disc brakes is the hydraulic modulation you can get. Not so much braking power.
The reason why race cars use ceramic is due to heat build up. Race cars can in extreme situations heat up steel rotors to temperatures that would ruin heat treatment and but more commonly they just greatly reduce performance as they heat up.