You don't want back pressure. What you want (and what most people mistake back pressure for) is velocity. In broad, general terms, the smaller the piping diameter, the faster the exhaust gasses will flow.
Think of blowing air out of your mouth with your mouth wide open, versus blowing out of a straw. Not much velocity to the air coming out of a wide open mouth, but from the straw it's quite different.
So what happens when you slap on a larger diameter exhaust system is that you lose velocity (and with it, low to mid-range torque) until the engine is moving enough air to properly use the larger diameter piping.
Tuning an exhaust system properly is quite difficult. Just look at how many updates F1 teams make to their exhausts. Bigger is usually NOT better, especially if you are not adding some sort of forced induction.
What you typically want to look for in an aftermarket exhaust is smooth bends and transitions, as well as smooth welds on the inside of the piping. Anything on the inside of the piping that can cause turbulence will hurt performance because air running through a tube travels along the walls, not the "empty space" on the inside.
As a (very) general rule of thumb, smaller diameter piping (and long header primaries) will help improve low to mid range torque, while larger diameter piping (and short header primaries) will help improve top end power. Unless you are drag racing, you want a system that favors low to mid-range torque because that's where you are spending the majority of your time. As much as we would like to think that we are speed demons that drive everywhere at WOT, very little time is actually spent at redline.
Hope that helps somewhat.
