At 5mm of thickness there appears to be "rivets" or some sort of metal pins appearing on the brake pad material.
This rivet could be the source of added "grinding" experienced by so many. Myself included.
Also of note, the brand new Red EBC pads are only 10mm thick.
Therefore, at around 50% thickness you get these rivets that start to scrape the rotors and could cause the grinding we all hear.