Actual benefits are hard to quantify. Everyone drives differently in differing ambient conditions.
The best measure is a before and after dyno session and most exhaust manufacturers post this info on their websites specific to the application and generally for catback systems only.
There are other variables, of course, such as whether or not there is an intake mod (CAI) and any special programming.
For illustration purposes only, here's my dyno sheet with a stock hemi run and then the addition of a K&N drop in filter and a Predator programmer. I am pretty sure I am not yet realizing the full benefit of those mods until I add a true CAI to the truck.
The Magnaflow single in/dual out muffler was already on the truck when the stock run was done.