I can find the SL 129 Calipers for $100.00 each. From the reserch I did, I found that 500E has the same ones. Batter brakes are always an added advantage. My 89 535i has 91 M5 brakes with Porterfield Carbon/Kevlar pads and, it was a huge improvement over stock brakes. I just want to do something similar to the 400E since I'm doing some performance upgrades to get more power.
As far as the Supersprint Muffler, It'a pretty quite. Similar Muffler on BMW gives a more throatier sound.