I wouldn't want any back pressure. Open and free flowing is the key. Tune them to your rpm range by changing diameter. You want velocity in the rpm range where you want power. If you want low rpm, then a smaller diameter will do that. Downside is they don't have the capacity to flow the volume of high RPMs. This creates back pressure, but only above the RPM you are looking for. A larger diameter will handle the volume of higher RPMs, but velocity will be lower at low RPM, costing you low rpm power.

Bottom line... Back pressure is bad. Velocity is good