Honestly none. But if you want to believe the scavenging theory when not all motors fire cylinders in the same order just be stupid like the rest. Who ever has one will never admit they waisted their money. I personally never saw a gain on anything but a ford which has a totally different firing order then Mopar or GM..
If you were buying one from me and were paying me to install it. There wouldn't be a better exhaust. And when its done you would spread the word that it is the best so not to look stupid. Then this would promote the sale of more X's to more ignorant buyers and I would make more money. I might even type up a dyno sheet for you to pass out.