I bought a direct OEM replacement cat for my 2001 Bonneville. I didn't have the time to put it on, so I went to Velaquez muffler, a larger chain of deep discount muffler shops in Illinois. They took a look at my direct OEM replacement CAT, and mentioned something about it likely would not work because it would have a seal problem. I paid $200 for the CAT at Rock Auto.
They ended up selling me a generic CAT for my Bonny, installed for $100. It has worked fine. It was half the cost, installed, of the OEM part. You may want to see if you have a local deep discount muffler shop that will install a generic CAT for a super low cost.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.