What is this part
 

I see a very rusty heat shield like part above the tail end of the cat - pics in this thread here:

http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/894010-exhaust-flanges-not-mated.html

Closest I found in the PET is 477 803 253 "transverse strut", called a "cover plate" in Pelican's catalog.

How's the installation?

That heat shield is a different part than the "transverse strut." the heat shield is held on with 4x 7mm or 8mm screws. The part you have the number for is held on with simple bolts. You'll be able to figure it out by looking at it