I would disagree.  A properly designed exhaust has low or no backpressure.
On the contrary, by taking advantage of resonant pressure waves, a vacuum
is actually created at the exhaust valves.  Essentially the pressure front
from the closing of one valve travels down the header pipe around the
collector and then back up the next primary tube .. and bounces off the
next exhaust valve about to open.  At that point you may actually have a
vacuum at the valve resulting in increased scavenging, decreased pumping
loss, and thus better efficiency.

I won't rehash the volumes of information out there on exhaust theory but
tube diameter, primary tube length and collector location all require
careful consideration to ensure both high gas velocity and the
aforementioned resonant effect.  It's important to note that this tuning is
generally focussed to a particular engine's planned power curve.

Check out the dyno tests at www.techtonicstuning.com

A quick intro to the theory:
http://www.powerflowsystems.com/review/12-00.shtml

A properly designed exhaust means better torque, hp, and mileage.  A good
example is my daily driver car.  It started life as a 90hp 1.8L VW Scirroco
that maxed at about 35mpg (imperial).  It now has a tuned 2.0 litre that
delivers 145 HP, yet still delivers 39mpg (imperial) at 60mph highway
cruise.  It's all about efficiency...and internal combustion engines
(particularly older ones) have lots of room for improvement.

My site http://my.tbaytel.net/guskers has a section on exhaust with
references to autospeed's muffler tests.  It is possible to have a quiet
exhaust with straight through mufflers if designed properly.  The exhaust I
designed has two straight through mufflers, one five inch OD, and one six
inch OD.  It is only slightly louder than stock...although the first design
with a smaller OD 2nd muffler was obnoxiously loud.