The fact that perceived safe speed and real safe speed may differ is not relevant. If we want people to drive slower, what's important is the perceived safe speed.
What is relevant is that people have a perceived safe speed, and that this perceived safe speed is able to be influenced. Change the environment and you will change the driver.
Some people are totally oblivious to whether they are safe or not and also reckless with the safety or other road users. This isn't some binary option there are far more factors at play than perceived safety. Even speed limiting cars on dangerous corners could have unforeseen safety implications in some situations.
I'm not suggesting to make poor quality roads. I am suggesting to not put up a 25mph sign on a road wide and straight enough to feel safe going 50mph. A main road near me is such a 2 lane road, 4 lanes widths wide, and is completely straight, but marked 25mph. It's no wonder most people go at least 10-15mph over. I do so unconsciously if I don't pay attention to the speedometer.
What is relevant is that people have a perceived safe speed, and that this perceived safe speed is able to be influenced. Change the environment and you will change the driver.