If you ever find yourself with a little extra time, it would be interesting to see what the Si muffler looks like inside. Over time there has been postings on the internals for the Z4M and pre-facelift Z4 boxes, but not the facelift cars.
Any plans for weight reduction, larger sway bars....? While everything is down you might want to consider reinforcing the front sway bar mounting area, see
http://www.zpost.com/forums/showthread.php?t=870503