Usually people upgrade the dp, that's it. When that happens, you create a bottle neck. What's worse is, the mp has a secondary CAT which is usually clogged or restricting.
Replacing the MP too helps a bit.
If you have a 3" exhaust system with a gutted CAT, you will see "some" gain going to a 2.5 or 3"...more in the spool of the turbo, espcially if you have a boost controller.
Bryan