Man, this has been an interesting thread.

Forgive me if I've missed any pertinent comments throughout, but I don't believe an engine needs any back pressure. However, with the reduction in backpressure comes an increase in horsepower and a decrease in torque.

I guess it is up to the individual application as to which is the optimal setup. If you want power high up in the rev range then little backpressure will most likely not be an issue.