I have raced many cars on real tracks and yes, what you say is true--for a race car.
On the street, simply removing the secondary cats is not going to make a car significantly faster. In my experience any increase under ten horsepower in a car that weighs a ton-and-a-half is not significant even to the butt dyno.
Now, if you replace the entire exhaust system and tune the ECU, sure you can get some noticeable gains, but that's not what we're talking about here and even so, I can't believe you're doing Mod Math.
Anyone chasing single-digit HP increases on a street car should really just learn how to drive better. You can't even accurately measure such small gains on a dyno--it's all within statistical error.