Easy way to figure it out is to engine brake with the muffler off. If it pops and sputters, the fueling is off.

This is not true

A free flowing exhaust will pop more because there is more oxygen for unburnt gas to combust with inside the exhaust pipe. Restricted pipes don't allow as much air in.

All stock exhaust will pop on decel. All exhaust pops on decel. It's just how fuel cut and timing works. Stock mufflers with catalytic converters are just really good at muffling it. It's what they're designed for.

Exhaust mods without intake mods have almost no affect on power. People report reduced power after an exhaust system because they're using their ears to gauge the throttle and ride more gingerly on their louder bike.

Back pressure is meaningless. A combustion motor is an air pump, why would you want to work against the air pump? The only thing exhaust geometry should be optimized for is scavenging.