A common fault on Boxsters is that although the stainless steel exhaust lasts for many years (mine is still good after 15 years) Porsche used cheap nuts to hold the manifold to the cat and they rust. This allows a gap to form and causes a horrible noise. The studs you need to remove are stuck fast. Here's a video of how I finally got them out.
https://www.youtube.com/watch?v=mITEttfNwAw
