With the e34 M5, removing the cat improves your 0-30 mph by one full second. I am currently building my own pipe. Frank charges $500 for the pipe and $150 for jet coating. I am making mine from Stainless and a friend with a shop is helping. total cost is under $100 and a case of Duvel. Add a conforti chip, and you have the only real mods that make sense.