[instarx]

An exhaust pipe in the airstream will have air travelling down the outside of the exhaust pipe and past the end of the pipe. This causes a low pressure area immediately beyond the end of the pipe due to the Bernoulli effect (turning a duct into an exhaust duct by blowing compressed air past it's open end is a common engineering trick when internal fans cannot be used, for example when exhausting explosive gases). Relocating the exhaust's end-point to a low pressure area behind the car would eliminate that particular pressure advantage. But which would be more effective... the Bernoulli drop or the low pressure area end-point? IMHO the difference in exhaust flow between the two would be so small as to be essentially immeasurable even with instrumentation.

A much more effective method to reduce exhaust back-pressure would be to install a larger diameter, straighter, or shorter exhaust pipe.