Its probably not the "back pressure" but the reflected pressure when the exhaust valves open at lower RPM.
"Back pressure" is a bad thing, anything that effects emptying a cylinder of all pressure will limit incoming fuel air mix on the intake stroke.
Often headers are tuned for low end OR high end torque, this has to do with the pressure waves that come out of the exhaust valves, at specific RPMs based on displacement, the reflected wave can be used to pull more exhaust out of the cylinder.
In your case you have changed the reflected wave point in the RPM, this should give you slightly more top end torque.
If you want to try and get more low end torque(losing any gained top end torque) then try adding a restricter pipe, smaller pipe to the end, or making your existing pipe longer.
Yes, that is more back pressure, lol.