There is truth to that. Engines need a certain amount of backpressure for enough exhaust velocity to ensure complete scavenging in the combustion chamber. A lot depends on the engine, its power levels, the target power band the engine builder wants..a lot of variables.

When you look for an exhaust system, that's one reason why manufacturers (factory engineers and aftermarket) offer several pipe diameters, header length, muffler designs, etc. Each is designed for specific purposes. The factory engineer generally goes for a bigger compromise as the engine has to perform over a wide range of conditions, plus meet emissions requirements, durability and warranty needs noise limits as well. An aftermarket engineer can make it for drag racing, high performance street, or any other more narrowly defined need.

Exhaust design is a science in itself. You need to define what your driving requirements will be to build an engine to best meet those needs...the same goes for an exhaust system.

 Think I remember for a street/ strip engine, the primary tubes size(headers) should be around 10% larger than the exhaust valve, and 29 to 30 inches long. For street driven, 31 to 32 in long. dont take this for granted,

1 5/8's O.D. primary's better low end torque a bit less top end HP, 1 3/4 primary's the torque #'s will be up the RPM scale with maybe a bit more top end HP, in your case with the 406 you are kinda on the bubble, which way to go, myself for a street strip setup with 2 1/2" exh.I would go smaller than larger. anips38655.4003125

Big pipes, cam, ports, intake, carb can give you great top end power, but it can also kill low end power and streetability.

You need to build the motor and drivetrain to match what you expect out of your car. The cool factor of high HP doesn't always make since if you are just concerned with your 0-60 times.

I've always been a 4-speed guy so I'am not really hip to torque coverter changes and trans mods, what rear gear do you have?

4.11's are plenty low enough, even if you only had 400 HP you should be able to fry the tires clean off the rims easy,????, have you checked to see if you have full throttle, take the air cleaner off with the engine off and wedge something against the gas pedal so its to the floor and see if the primary throttle blades are fully open, the carpet might not be allowing the pedal from going to the floor enough,if you have a vac. secondary carb, I'am guessing its a Holley,with the primaries fully open manually operate the secondary throttle blades to make sure they are not hanging up, don't try checking the secondaries by buzzing the motor to 7K from idle, its not made to work that way, the car has to be under power for them to open ,if the car runs and idles well and if its in the carb the only other thing it would be is a hole in the secondary diaphram or to stiff a diaphram spring, I believe the yellow spring works the best.
    BTW, is the GP the super charged model, also I believe the GP is front wheel drive, you will get a different feel of power or torque when being pulled as in the GP than being pushed as in the rear wheel drive Vette.anips38660.9587847222