IIII, it MAY be doable, but why pay all that extra money for parts you already have. Plus I'm not sure if the 4.8iS pipes would line up with the headers or not so, just buy the mufflers and pipes connected to them.
Cheap to modify and usually done in about 1 hour I hear.

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S