is there a hp diffrence with tailpipes vs. turndowns

[84LX89GT]

You may find top end power...but will it reduce low rpm torque. Are you talking about just race engines? On a race engine you are tailoring the power band for maximum high RPM power and torque and not worrying about gas mileage or engine efficiency at lower rpm's. backpressure brings the exhaust pressure waves closer together and can be used to tune the engine to a certain power band. The effects of backpressure as a positive are overruled when you go to WOT with a high flowing engine at higher RPM's.
You can make cars go much faster by almost eliminating back pressure, but some heavier cars and low hp engines don't respond well to eliminating all backpressure. I don't think backpressure is near as important as pipe diameter, tube length, crossover locations. I'm not talking about having enough backpressure to push exhaust back into the combustion chamber (like log style exhaust manifolds to), but rather a slight opposition to absolute free flow that can be used as an aid in tuning the airflow/scavanging of the engine.