Back pressure is never good for an engine. What is good for an engine is to get the exhaust gasses out as efficiently as possible. A smaller pipe can deliver more torque than a larger pipe and that is usually mistaken as back pressure. Internal combustion engine exhaust gasses flow in pulses due to the individual firing of the cylinders. If a pipe is too large, these pulses in the exhaust pipe can stop or even instantaneously flow in the wrong direction. This results in inefficient flow of the gas and poor performance along with intermittent back pressure. Exhaust velocity is what is important, not back pressure.
If you use a pipe that is too small it will result in back constant back pressure and will choke the engine. The velocity of the gas will be high but will not be high enough to let the engine breath.
Every engine has an exhaust pipe size that is optimal for it. An open exhaust will usually let the engine breath better but sometimes a muffler can enhance low end torque by smoothing out the exhaust pulses along with other issues that are more complex.
I run an open exhaust in my race car, when I can, to maximize top end horse power. However, when I am required to run a silencer (single camber race flow master), I do get a slight increase in mid range torque to slightly offset the decrease in top end power. This has been confirmed on the dyno and has nothing to do with back pressure. I have been building my own race engines for over 20 years and have learned a lot from some very knowledgeable people during that time.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.