Thats NOT strictly true
Turbo'd cars thrive of no back pressure but N/A cars dont always agree with it but some do.
Impreza de-catted & straight through exhaust will give approx 10% power gain.
BMW 325i De-Catted & Straight through exhaust will loos approx 10% power
Its worth doing your homework before going straight through.
You can still have a loud exhaust with silencers just use a singled baffle silencer instead of a typical twin baffled silencer.
Catalysts also provide back pressure.
Hope this helps ??