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camshafts, valve trains, etc. are designed to have at least 14.7 psi absolute backpressure. The goal is to open the exhaust valve late enough to get maximum thrust from the power stroke, but early enough to get the exhaust flowing so the intake can get fresh charge.
if you suck out the exhaust, you might lose a bit of thrust.
Further, if you leave a bit of exhaust gas in the chamber, there will be less pumping loss on the intake stroke at part throttle.
is it a big deal? Nope, but if you are looking for the next 5 to 10 percent efficiency at part throttle, it can add up.
also, what is going to provide the energy to create the vacuum for the exhaust?
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