The reason that some people say you need back pressureis mainly because they've always heard you need back pressure. The people that say you don't need it believe in efficiency of air flow.
The theory mainly behind the need of back pressure (correct me if I'm wrong. An old senile mechanic told me this) is on the older cars the air and the fuel wouldn't mix all of the way in the cylinder, so the surging back pressure on the other side of the exhaust valve would form a sort of "swirling" effect inside of the cylinder to properly mix the gas and the air. With newer engines, there are systems that usually will compensate if there is unburnt fuel in the system (which would be a problem with no back pressure on older engines).
Another misconception is people confusing back pressure and proper air flow. In a bigger pipe, air flows slower than the same volume of air in a smaller pipe. This velocity is needed for the bernoulli's principle (suction is made when air is pulled over an open hole) to be applied at the exhaust valves to pull exhaust out and intake in during overlap of the valves. Bigger pipes= slower flow= less suction due to good ole bernoulli
So to answer your question directly, you'll be fine, just might sound like a weed eater with a 2.9
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