There are a few things I can think of.
I had a similar experience with the plastic backing. It was (and is still) loose on one of the corners. Sometimes a hard enough stop causes it to slip and it feels like something has hit the seat and makes a thump sound. Whatever the plastic is catching on (perhaps a partially broken clip), it does this on hard stops for me from time to time. So, check to see if the backing is secure.
Otherwise, I wonder if your lumbar support is loose. Can you adjust it properly?
Finally, It might be worth carefully removing the rear plastic covers from the seat rails to see if the bolts are fully secure.
Other than that, I can't think of anything. The separated backing seems legit, though.
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
"If you want to criticize someone else's work, be sure that your own is in order first."