The word back pressure is used a couple of times in this thread.My opinion is there should
be little to no back pressure in the exhaust system. The pipe should be small enough to maintain exhaust velocity to help evacuate the
cylinder. I think when you went to the larger pipe it probably slowed the exhaust gases down enough to effect low end torque by not evacuating
the cylinders as well at low RPM.
When the exhaust valve opens the gases escape at
a high rate of speed as the piston rises to near TDC the gases start to slow down. If the pipe is
small enough or large enough to maintain velocity it will help
to pull the remaining gases from the cyl.
I also believe that a cam with a lot of overlap
and an exhaust system piping to small will lead
to over scavaging the cyl.(pull to much of the
incoming charge thru the exhaust valve)leading
to poor performance.
I guess what I am trying to say is the pipe dia.
should match the RPM range you want run.

My opinions only
Arnie