Yes. Although I have never taken the time to actually test a few of these, everyone who removes the cat and muffler reports more SOTP power, quicker spoolup, and lower EGT.
The cat (IMO) was a bad idea, particularly for some of these truck that get used as daily drivers. Implimentation pf a gas technology on the oil burner was a sad attempt to get an early emmissions certification. They are prone to loading up when they don't get hot.