Took my car to the muffler shop to have the x-pipe installed. Interestingly the inlet side of the muffler is 2 1/8" and the outlet is 2". Bought the 2 1/4" Magnaflow x-pipe and with a little flaring of added tubing etc. the install went well. Note to others considering this mod - the couplings post manifolds need to be loosened to drop the exhaust pipes enough to weld around the tops. Ensure the person doing this pulls the excess cable for the 02 sensors BEFORE he drops the pipes. Thankfully I was watching & helping during the process (most are not permitted in the workshop area but it was a friend who did the job) so immediately saw what was about to happen and pulled the slack before anything broke.
The 4.4i sound quite nice now, with a bit of rasp to the exhaust note, not dissimilar to the Corsa system I have on my 6.1 Hemi. No noticeable drone from inside the cabin, but a definite 'note' from outside. Very happy!

__________________
08/06 build 4.4i Stirling Grey over black