Alan, just go to M5board daily in the for sale secition. These cars have become old enough that you can find many decent used parts and exhausts from credible sellers, where you don't have to pay retail on a new one.
+1 bought and sold all my parts off there.
RPi's GT exhaust kinda rocks too btw...oh, add scoops!
Here's the big secret that makes the 20 way BMW seats fantastic (I had them in my E38 and briefly owned E65 as well - missed them a ton when I had the 335i).
Adjustable shoulder support!
I tend to sit fairly upright and a little hunched over when I drive ("the active fat man position" as I like to call it as opposed to "the lazy fatman position" where you recline so far back in the seat you can rest a beer can on your gut). Thus in a conventional seat I'm supporting my whole upper body - as the seat back, slightly angled, loses contact with my back around nipple level. With the adjustable shoulder support, I can tilt that top part of the seatback forward and support my shoulders and thus take all the weight off my back. Very, very comfortable.
The other 18 ways include the lumbar support (six ways), the thigh support (two ways), width adjustment (two ways), height (two ways), tilt (two ways), seat angle (two ways) and headrest (two ways). These are all nice, but its the adjustable shoulder support that gets me all excited when I think about my BMW's seats.
Did you have active seats in yours NB? Pretty cool feature.
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2008 BMW M5 6-speed
Silverstone II/Indianapolis Red/Aluminum
Enhanced Audio, 20-way seats with Active Backrest, iPod, Comfort Access, Shades