Originally Posted by Meeni
Are you sure you understand the difference between a muffler and a catalytic converter (cat) ? If you remove the cat, you will NOT pass emission test, NOx will be out the window and will fail you (not all states have emission test, though). This is a box that contains chemicals, if it gets abused (overhead, contamination from coolant/oil additive), it gets damaged and needs to be changed for much $$$.
Now it seems the flap has nothing to do with the cat (heating it or not) and that running the cat cold for extended period of time has no effect on it (checked myself since I got ironic answers here... ).
its actually seems to me that you have no idea what you are talking. Many people remove cat converters for performance benefits (called "test pipe" mod), and I have done so on 2 of my cars. Both passed inspection just fine. I believe only CA has different emission regulations which MIGHT prevent you from doing test pipe on some cars
The way cat converter works is when its cold it simply passes much more toxic crap, and it only works efficiently when its running hot. Sooner its warmed up, better it is for environment, and better it is for BMW because now 335i runs greener.