Cat-back is just that - everything behind the cat(s)- mid pipe, muffler and tail pipe. If you need to replace the cat, I can share my experience with Bosal units.
I had to replace both of my cats earlier this year, the originals were shot w/ 200K on them. I'm familiar with Bosal in regards to replacement exhaust components for European cars, so I gave them a shot for the cats on my T4R.
Rockauto had them for the best price, and one of the cats was a "premium" unit. They fit up really well and have been flawless so far. I saw the walker replacement units, they just looked cheaply made, no comparison to the Bosal units. Just my .02 :cheers: