Can anyone provide me links to flowcharts for cat back systems? I want like Flowmaster, Borla, Random Tech, GMMG, all that stuff because I want to compare the flow between them.

 

I can honestly say that in 15 years of performance parts sales I have never seen a complete flowtest on an exhaust system provided by the manufactuer. Not that it would be a bad thing.
Wave cancellation mufflers such as the flowmaster do not flow well on a bench. They rely on exhaust pulses to function properly. I used to have a poster of a Chevelle on its back bumper and a flowbench sitting in the lane across from it. They said something like, "which would you rather race?" or or something like that. In other words the flowbench is not the benchmark, performance is.
The Borlas I have owned were supposed to be a 3" muffler but the internal flow tube was 2 3/4". Borla told me that they must have been built on a friday. hahaha.