This will put this issue to rest definitively.
I have a Chevy 350 V8 with carb. I'm running custom headers, Y pipe, Magnaflow cat, and stock S13 catback.
The red torque curve is on stock exhaust. The Edelbrock was dialed to the leanest setting and AFR was only 11, which is extremely rich for an NA.
The blue torque curve is on open catback. Notice the huge jump on power. AFR was at a healthier 13.2, not perfect but almost ideal.
Now I realize that this doesn't apply to KAs, but it shows how restrictive the stock exhaust is. There is too much back pressure and the cylinders can't clear exhaust gas = incomplete combustion = rich.
