I did not read this entire thread but the few pages I did it would seem several had the question of, "what does the flap in the exhaust do?"
When you start the car it closes to help bring the o2 sensors up to temp. Then opens after x temp is reached on my E46. I would think it does the same here. None of my after market cats have had one that I know of.
Thanks to: Hixson BMW, Race-Keeper, Koni, BimmerHaus, & Hoosier
2012 SUMMER NATIONALS SCCA PRO-SOLO BONUS CHALLENGE WINNER
2012 SUMMER NATIONALS SCCA PRO-SOLO A STOCK CHAMPION
2012 SCCA DIXIE A STOCK CHAMPION