Since you’re not convinced, what do you think the slots do to improve braking?
I do not think it, here are the facts:
1. The slots in the disk surface “clean” the pad in the braking phase, regenerating the friction material, removing debris or glaze. Downside, pads may not last as long.
2. The slots disrupt the layer of water on the disk when driving in the rain. The layer of water is what makes "wet" braking so ineffective.
3. The slots guarantee dissipation of the gases (fading effect) during extreme braking
Your assumption that cold rotors will not result in fading during hard braking is wrong. The affected area is the pad to disk interface. The thin area of the pad has to convert the kinetic energy of the vehicle to thermal energy via friction. Deceleration is a squared function of time. So if you half the time, the thermal energy that is generated is increased by four times (not exactly but close.) During hard braking on cold rotors this thin interface can overheat and gas. The extra area on the disk from eliminating the slots is useless if there is gassing and the pads lose contact with the disk.
By the way, the pads do not ride directly on the metal of the rotors after the brakes are bedded in. The pads actually work with the thin layer of pad that is deposited on the disk. That is why new brakes should be bed in by using the bedding process.
Like I said, if one rarely tows or hauls heavy loads, the factory rotors are more than adequate. I stand by that. When I retired and rarely tow heavy loads, I put flat rotors on my truck when I could not find quality slotted rotors. I have noticed the difference a few times, mostly in the rain. I do still have slotted rotors on my 1969 Nova drag car and my 1974 Laguna S-3, a heavy, fast car, with a 5 speed and a 540.