Backpressure? Any backpressure is bad. What you have to worry about is exhaust gas velocity. How much backpressure do you think top fuel drag cars have? Very, very little. Don't you think if backpressure were needed, of all cars, top fuel dragsters would encorporate some?

Let's further discuss EGV (exhaust gas velocity). Wonder why some Hondas have a higher pitch that others? Maybe they have an aftermarket muffler on the stock exhaust tubing. It all comes down to exhaust size. Too small, as the case in most stock Hondas, will choke your engine. Imagine trying to blow through a coffee straw/stirrer.How long does it take you to run out of air? 45 seconds? 60 seconds? Longer?

Now let's look at the other end of the spectrum -- too large exhausts. Blow with the same blowing force as applied when blowing through the coffee straw, only this time blow into nothing, just air. Takes you, what 2 seconds to run out of breath. An exhaust that is too large will also choke the engine. Why? Well, because the exhaust piping is too big, the exhaust gas isn't compressed very much and therefore travel at a slower pace. Sometimes too slow. In this case the gasses get backed up in the system and choke the engine. Also normally results in a lower exhaust note.

Finding the happy meadium between the optimal exhaust gas velocity and pipe size can be a fine art, but it's only as hard as you make it. A good rule of thumb is minimum 3" exhaust pipe size with turbo cars, around 2.5" for n/a cars making at or under like 250hp at the wheels.

In conclusion, backpressure = bad, exhaust gas velocity = good.

In relation to your original post, the reason less 'backpressure' hurts low-end torque, is not because of the loss of backpressure, it's because the engine isn't flowing enough air at lower rpms to effectively move all the exhaust gas out of the system. The EGV is too low.