| Felix: What you say is true - for a given deceleration rate and moving mass the amount of kinetic energy converted to heat is the same for iron and steel rotors.
However when you use iron, the frictional force that is generated is higher for any applied piston force (F=uN). So, because of the higher maximum frictional force that can be generated with the maximum piston force available, the maximum heat that can be generated is higher.
Consequently, under maximum bike deceleration (which is higher for an iron rotor, i.e. better brakes) an iron rotor gets hotter than a stainless rotor does at its lower maximum deceleration rate.
Only when you get below the maximum deceleration rate achievable for stainless is when the rotor heat load is the same for both iron and stainless. | 
Shazaam!
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