[dsfyu404ed]

>ports are shaped to work with the back pressure provided by a working cat.

This is a 100% urban legend BS myth.

There's a partial exception for two-strokes where if you tune the system right you get a resonance wave that lets you pack a little more mixture in on the intake/exhaust stroke. Literally no 4-stroke ICE benefits from back-pressure in any way. It's just that sometimes the things done to reduce back-pressure (small exhaust pipes being the primary one) seem like they would increase it and vice-versa hence the urban legend.

Edit: Who the heck is downvoting this? This is how ICEs work. Google it if you don't believe me.