True! Exhaust gasses via our internal combustion engines are different. To much going on to nut shell excellence.
The HVAC comparison should be taken as "kind of like" to help get a message across.
The workings of the engine from air cleaner to exhaust tip are all involved in some way shape and form.
I order to make things easier, when asking an exhaust sizing question, the more engine info given along with vehicles intended useage is the information needed. You don't have to go nuts listing everything, the basic HP/Tq outputs with vehicles intended useage is enough to get a exhaust tube size.
When it comes to mufflers, there's only two ways to choose a muffler.
Sound or performance. Performance should be the choice and goal.
Should the sound meet performance goals, then all the better.