I have read backpressure here and it really is more or less a myth with four stroke engines, two stroke engine does need back pressure but with four stroke fuel injected engine there is no use for that really.
The truth about exhaust backpressure and torque
Good exhaust manifold for economy is built so that length of pipes matches cruising rpm, each pipe should be equal length too, it is made so that previous exhaust pulse 'draws' next one out, so that cylinder fills up easier with fresh stuff and with length of pipes you can match effect to be most at any given rpm. There are surely lot of calculators for this too, haven't bothered to look that much.
It is similar myth to one where intake manifold should never be polished, that was because with carburetor fuel was mixed better with air when there were some turbulence and fuel did not get stuck to intake walls when there was rough finish, but with port injection there is no need for such, so intake can be polished for maximum air flow.
There are lot of myths with cars and especially with performance. Many performance aspect also increase efficiency of engine as basic idea of adding performance is to get more air into cylinder, performance minded will put more fuel to the mix, one that likes to get more mileage out from car wants more air into cylinder to burn engine bit leaner, however be careful not to overdo that, another benefit of better flow trough engine is that it will burn cleaner.
I did read from somewhere how somebody seriously was worried others messing with intake pulse ram effect by making changes to airbox, now there is one misconception again, that stuff should be left inside to intake plenum, certainly airbox has nothing to do with that effect, but so strong are beliefs instead of scientific facts that beliefs live and spread more.
Diesels are much simpler in there as they are made to burn lean, they are made to 'knock' so those are not easily broken by going really lean.