One of the biggest reasons to use stepped primaries has little to do with pulse wave formation. It has more to do with gas density.
As the exhaust gas cools while moving away from the cylinder head it becomes more dense. A larger pipe cross section allows the "slug" of exhaust gas to keep moving...
Exhaust extractors have been around for a long time.They were popular for british sports car going back into the 1950s. A search for the Alextractor by Alexander Engineering Co. should give some useful information.
They work as you describe above, using air movement under car.
