It’s made a huge improvement — I have since driven 1,300 km over a couple of days with minimal fatigue or soreness. The back of the cushion no longer feels like it's sinking.
I added a sheet of 1/16" thick plastic between the cushion and suspension mat. I immediately felt more support; however, I haven't driven for any extensive amount of time.
I’m experiencing a similar issue with my Lariat. There isn’t enough support at the rear of the cushion, and the sinking causes my butt to go numb if I drive for more than 30 minutes. If you reach underneath the passenger seat, you can feel a thin piece of plastic sitting on top of the mat...