is there a hp diffrence with tailpipes vs. turndowns

[Ron1]

I will ask again...what makes a "lil" bit of back pressure a good thing? You tune your exhaust for what you have. Any resistance in pushing the exhaust gasses out, robs power. And how do you build back pressure? Start putting some crimps in the tail pipes until you measure a "lil" bit?
Do you try 5 different exhaust systems until you get what you think is an adequate amount of back pressure? And if so, what is that? A "bunch" of torque? Dyno sheets by any chance?

Ron