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Well, when forced induction is applied, back pressure essentially becomes your enemy (typically caused by catalytic converters, mufflers, etc...).  Eliminating as much back pressure as possible allows the possibility of running higher boost pressures, in turn, increasing the horsepower output of the engine.  Also, Naturally Aspirated engines need SOME back pressure to function correctly.  Hope that helps.  Personally, I would have to keep mufflers on the car if it was mine (I like quiet road trips).   Feel free to correct me if any of my information is incorrect.