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think about the term of "backpressure" itself.... it's pressure(which itself is a measure of restriction to flow) between the tip of the tailpipe and the beginning of the exhaust port in the head.
resistance there is simply more work that the piston will have to push against to get the exhaust out of the combustion chamber. if anything, "backvacuum" would be beneficial to efficiency.... it would literally suck the piston up during the exhaust stroke. this exists in a relative sense, go from a sea-level altitude to a mile up... the volumetric efficiency of an engine actually rises due to less force from the atmosphere pushing exhaust gasses back into the combustion chamber. of course, since the air is significantly less dense, the engine doesn't create as much output either.
a lot of the times, when people install exhaust systems that "lower backpressure" and see/feel a hit to power/economy at lower RPMs, it's because they lowered backpressure at higher RPMs..... and raised it at lower speeds. an engine is a series of compromises, if you desire to tweak them out of factory specs, you'll get out of factory results.
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