Backpressure is not a myth, maybe a poor term to describe what is going on, but not a myth.
In an exaggerated case, Open headers allow tremendous flow out of an exhaust valve at high engine rpm, where scavenging really isn't a necessity. On the other hand open headers don't allow the the airflow velocity necessary for maximum power at low-mid range rpm where scavenging is necessary for maximum torque which directly relates to hp.
Even the best exhaust system is a compromise, and is tuned for optimum performance at a given rpm on a given engine.
Obviously, airflow obstructions like catalytic converters and restrictive resonators are not the best for maximum flow, or keeping up the exhaust gas velocity, but when the air goes from a small pipe to a large open chamber (like a gutted cat), then back down to a smaller pipe, velocity is really slowed down, though maximum flow may increase. Now the question is, do you gain more from the increased flow than you lose from the decreased velocity? sometimes yes, sometimes no, sometimes its a wash, it is dependant on too many other things to give a definative answer.
I guess the bottom line is, if you want maximum peak power at high rpm, go as big as you want on your exhaust, but if you don't want to kill you low end, keep it a bit conservative (of course as smooth as possible too) and maintain your exhaust gas velocity.
Personally, I'll stick w/ a bit of a conservative approach, simply because my street driven SVT doesn't spend too much of its life at redline
