restriction is necessary to some degree to help build torque at certain RPM
That's really a very common misconception. Restriction never helps. I think the reason this idea got such big traction is from putting on 'restrictive' exhaust pipes and headers, only to discover that engines make more power that way sometimes.
Gas flow dynamics have everything to do with compression waves. If you can get good velocity, and have your exhaust designed right, then the soundwaves (pulses) in your exhaust really help to suck the next exhaust stroke out of the cylinder, at certain RPMs. A giant sewer pipe exhaust where the spent gases can exit freely, but at low velocity, will never have much effect on scavenging.
Don't believe me? Stick a straw in your mouth, and blow through it, hard. Stick your tongue on the end to stop the air. You can definitely feel the rapidly moving air in the straw try to 'suck' your tongue down the tube as it all comes to a stop! Now try the same thing with a paper towel tube. Heh. Even if your tongue were big enough to pull that off, you just can't get a lot of velocity going, unless maybe you're Superman.
So in effect, it's not that a 'restrictive exhaust' is helping to make power - it's more that your exhaust will often respond much better to a 'just right' size, or even slightly too-small, instead of being too big. Especially if you tune the lengths, and run a crossover. Your average power will go way up, even if the 'big pipes' give you more top end.
Another thing that really freaks people out is how a glasspack muffler can actually flow more than a straight section of pipe with the same internal diameter. The rough walls should slow things down, and create drag inside the muffler, right? Well, the thing is, as the exhaust moves through a 'smooth' piece of pipe, the outside edges grab the walls of the pipe and swirl around, creating vortices and drag. When you shove gases through faster, the effect gets more pronounced.
In a glasspack muffler, the perforations in the tube not only hang onto the air, but high-pressure turbulence just falls through the holes (for lack of a better description) instead of being forced back to the middle, creating restriction. As a result, you get very smooth laminar flow. The slow, low-pressure gases clinging to the edges of the perforated tube allow the column of gas in the middle to blow through with much less restriction. Granted, the old glasspacks (and turbo mufflers too) are a bit loud, but there's a good reason they have always been popular since they were introduced. They work without decreasing performance! =)