Uhhh... NO.
Most muffler shop guys I've met are just guys that can bend pipe and weld it together. Most do not race or care to race. They only know what they read in the trade magazines.
Cylinders dependant on backpressure? NO again. All engines make more power with less restriction. I laugh when I hear old timers say that their 340s and 440s with Magnum manifolds make as much power as the same engine with headers. Faster in, faster out, MORE power.
A stock engine will lose a little bit of power at low speeds with large headers, but take that same engine with iron exhaust manifolds and do tests on power with then without mufflers. The engine will make more power with less restriction.
Getting back on topic: 2 1/2" exhaust works fine up to around 425-450 HP. Beyond that, you will see benefits from going to a 3" system. I had a 2 1/2" system put in my Charger when I swapped from the stock 318 to my first 440. The 440 made around 380 HP and the car ran great. Currently I have a 440 based 493 and my 2 1/2" pipes were too small for my combination. I went with a 3" mandrel bent center section with an X crossover along with 3" in/out mufflers, yet retained the 2 1/2" tailpipes. Power jumped up dramatically. You'll find that tailpipes can be smaller than the center section with no loss in power. The exhaust gases have cooled off some at that point to where large tailpipes are not as important.