I was gonna respond deuce-anda-quarter, but knew my memory all leaked out a while ago.

fwiw, you already have the best exhaust from BMW, you might wanna read up on people's experience and reaction to what they DID TO their X (some unhappy stories).
a search on Flowmaster alone was 38 hits, might wanna check eisman, and definitely U-Boob for all the variations that have been tried, complete with cheap camcorder sound.

in my reading, Eisman has the most 'yep, I heard it/lived with it.. def best' comments, though I may have missed a couple in my exhaust upgrade searching. Else? 4.6 / 4.8is has best sound.
If there is not a "fits bmw e53" application by NOW, aren't you kinda' askin' for trouble?

__________________
carpe` cevesi
knf

01 topaz e53 3i 5m 120k mi =
$9.7K + $2.4K when seller says "needs nothing" (131K)

Drew Cary: I love driving stewpid in hella thunderstorms.. I roll stop-signs.. get pulled over, the high-school cop says, "Do you know why I pulled you over?"
I say, "Sure.. do you know why I did it?"

2012 Election: Congress has determined that government beatings of all taxpayers must continue until morale has improved

Gone: '67 Galaxie LTD 2D aqua, 72 Pinto, 76 Elite, e12 track.. long road