shiv was cool enough to show us this dyno plot comparing his Stage 0 configuration, and what happened when he threw on a catback (SPT optional muffler)
The highest HP run is with the muffler
You can see that the range in which the catback showed improvement is from about 4500 to 6500, and it's not a huge increase, but a reasonable increase. This is already with all the improvements of Stage 0, so I'd guess that the effect is larger than it would be with the stock exhaust tract.
I'd expect it would have a bigger improvement if a less restrictive downpipe were used. However, in all cases I'd expect gains primarily in the higher RPM range...4000 - 4500 RPM and up. No real gains in the lower half of the RPM range.