normally when you look at HP and flow numbers from the more know exhaust companies they are all good so that is why people check down to sound, fitment, drone, etc.
IE billy boat, Coarsa, Magnaflow, Borla there is not much differnce.
biggest gains will come from longtubes first then catback second.
1.borla flows awesome but will drone
2. Corsa works well does not drone but i don't like the sound at idle as much as borla
3. magnaflow uses 409 pipe so I don't use them
4. billy boat is a dual mode exhaust like factory so you can tone it down if need be.
5. stinless works sounds great with there headers but some have complained of fitment issues.
my .02 is start with headers and if you still want catback go from there.
My buddie just did heads and cams on his Z06 and the Billy boat system was a god send on calming down the drone as it went off the charts bad.
2013 ZL1 -sold
2013 Megacab 8" lift 38"mt's