I'm 6'4" and what usually gets me for headroom is the sunroof frame, which will sit just above my head and does not allow the sunroof to provide any extra useful headspace. However, the Model 3 roofline appears to allow the driver's head more room.
This Model 3 test drive VR video shows the headroom available for front and rear passengers. For tall rear seat passengers, it looks like bumpy roads might make them hit the
side of their head on the roof frame, as it does appear to be close. Don't forget to move the camera around!