The newer style rotors like Shimano Ice with aluminum spiders and aluminum sandwiched in the rotor dissipate heat better. I use both SS and AL-SS rotors on different wheelsets, and definitely notice the difference. Newer pads and larger rotors are a better at heat dissipation.

The real concern is not the rotor heating up to the touch, which is normal, but brake fade and boiling hydraulic fluid from the brakes overheating.