I went a somewhat different approach on my 69 Z. I built the engine to conform to PS rules which also limits exhaust upgrades stating 2 1/2" maximum diameter, stock muffler configuration, and "H" and "X" cross overs are allowed.
I used the Pypes system. Stainless steel, "X" cross over, mandrel bends, and stock transverse muffler configuration. However I took it a step further and spent the time grafting stock reproduction 2" tailpipes coming out of the muffler so when looking at the car, the system appears stock.
Here are sound clips
This car still had it's born with exhaust system with just the muffler being changed to another GM deep tone replacement. At this point the muffler is already over 30 years old so I think it's somewhat blown out and raspy.
https://youtu.be/SDMeBoQpf58
Here is the new Pypes system with their 2 1/2" tail pipes
https://youtu.be/5dTNw93VPEQ
Here is the system after I grafted in reproduction 2" tailpipes for a stock appearance, and it actually tamed down the noise a bit which was a bonus.
https://youtu.be/IZ0HlEY7b74
I can tell you from my own testing, I have another F-body that I also run in PS. I started racing that one with a reproduction Gardner exhaust system (transverse muffler) I later changed that car to a Pypes SGF70 transverse exhaust system as well (like above) and with no other changes to the car at all, it picked up 4 tenths ET and almost 4 mph. Some of the gain is the advantage of mandrel bends, "X" cross over, and the slightly larger 2 1/2" pipe, but the muffler was also a big improvement with a straight through design. As far as transverse mufflers go, the Pypes, and the Magnaflow replacement are both similarly designed and actually flow exceptionally well. Hope that helps.