Why open up your exhaust only to choke it at the cat?
I have a friend who's running Borlas with a stock cat, and a STI cat back. He aslo has a Ganzflo intake, bored out T/B, upgraded clutch and light flywheel.
On the other hand I have a Cobb intake, and Full MRT exhaust. (high flow cat)
He and I run almost identical times in the quarter mile but I gain on him moderatly quickly in 4th gear.....
and he has a fly wheel while I'm still running the stock ship anchor on mine.
We're guessing that the stock cat is really holding him back.