This is all over the map:http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/
I come across this information a lot. which sums up around 2.2cfm per HP often.
I'd like to see more on this. Honestly I find this really hard to believe as possible. I don't know how the E90 exhaust is, but my E89 N54 exhaust, especially the passenger side pipe, was a complete kinked up joke.There is also a post on another forum that I can't find that Payam said he measured no difference in backpressure with stock exhaust vs no exhaust (if I recall correctly) on 700+ HP.