Unfortunately, I'm not aware of any such feature on the IS (nor have I read about one).
But if its not too far off topic, can you tell me more about the BMW feature? It sounds like you're describing an exhaust "cut-out". I didn't know BMW's had this either. Is it a stock feature or possibly an aftermarket add-on? Lastly, is it located before the muffler or on the muffler?
I will say that cut-out's are typically absurdly loud (and at all RPM's, not just idle), so perhaps what you're referring to is some sort of resonator for certain frequencies. Kind of makes me think of the soup can mufflers (except I haven't seen one on any BMW's...not that it hasn't been done!).
Man im not sure if i am explaining it to you right, since my friend has the 335 with the sports package... What he told me is that it has something to do with the muffler, and how a valve was closed at idle and at low rpms by unplugging the valve [something like that] it is always open therefore creates more noise...