It's not actually back pressure you are after, it's gas flow. You need to make sure the gasses flow freely to aid in "scavenging", back pressure does not help you at all. The faster the gasses flow, the easier your engine will clear the cylinder on the exhaust stroke, meaning the motor does not need to use the the momentum from the power stroke to push the gasses out. This does not mean a massive pipe will mean better flow. Think of it like your garden hose. If you had a 4" hose with the normal amount of water flow, it will dribble out the end and not be a steady flow. When you use the correct size hose, the water flows out much faster and steadier. This is what you want to achieve. If you put your thumb over the end of the hose, the water cannot escape as easily so comes out at a higher speed, but the actual water flow slows down because of the restriction. This would be "back pressure" and is not good.
This can also be taken to a whole new level by tuning your exhaust to the extent of it creating a vacuum and actually sucking the exhaust gasses out of the cylinder, sometimes referred to as negative supercharging or vacuum supercharging. Read here for more info
http://www.impulsengine.com/
But yes, a premade set of extractors and exhaust system like that will allow you engine to breathe better and you will notice a difference. Getting your carb/s tuned at the same time will aid more in performance and economy, Especially as your intake manifold will be changed as well.
Cheers,