You cannot compare forced induction to N/A motors when speaking of exhaust modifications. They are COMPLETELY different, especially the turbo.

 

Let me elaborate on that. The forced induction vehicles have LOTS more air and fuel going into the motor, and LOTS more exhaust coming out. The increased pressure from the larger mass of gas is enough to force through the gutted pipe to show a slight to moderate increase in whp, 1/4 time. Compare this to the N/A motor, where the much smaller flow of air is going to be much more disrupted by the turbulence of the gutted pipe.

 

I think gutted cats etc will net better gains on FI cars vs NA cars. I see your point. But that is not to say any negative effect occurs when NAs do it. How do you explain the LS1 guys netting gains with this mod (na)?

I've NEVER heard any of my friends or co-workers complain of power losses from gutted cats. Maybe clogged cats

 

Gut the resonator on any stock exhaust or many aftermarket ones and you will see it is nothing but an expansion chamber in the tubing. It looks a lot like a can with two pipes coming out the top and bottom. I guess some people on the internet know more than exhaust tuners worldwide. I believe Nissan engineers used a flowbench to tune the motor on a bench dyno. Gutting the filler in their system only reduces unwanted backpressure. My 0-60 via G-Tech Pro is 5.0 seconds flat. Must be from the power loss based on your internet exhaust design theory. I also dynoed more torque and HP on an '88 Porsche 944 going from a Bursch test pipeto a gutted Porsche cat.