I am NOT an fluid-dynamics engineer, nor am I a tuning expert who makes his living by building exhaust systems or by tuning them.
When designed to have the power stroke of the internal combustion cycle for cylinders closely-follow one-another, the increased exhaust pipe pressure creates a relative negative pressure compared to the later-firing cylinder and its exhaust. When the following cylinder fires, it has its exhaust gases "pulled-along" because of that pressure differential. That's scavenging.
Done correctly, the timing of the exhaust impulses, coupled with the diameter of the pipes, can reinforce the relative efficiency of the exhaust flow. The operational rpm range of the engine's design, the need for power vs. torque, the position of the powerband, all these are factors in the exhaust design.
Small exhaust ports promote faster-flow, larger exhaust ports promote more flow. Because back-pressure helps to keep the incoming fuel-air flow in the cylinder, awaiting combustion, the size of the exhaust passage and the valve(s), and the desired operational parameters of the engine, such-as torque across a wide range (what you want on the street) vs. the 'best' power (what you probably want in a racing engine where the engine commonly operates in a narrow range of its total rpm capacity, like a dragster, or a track car/bike) all are taken into-account when designing an efficient exhaust for a particular use.
Equal-length headers, when properly sequenced for the gas flow, will have this 'scavenging' inherent in their design. Four-into-ones are typically balanced towards higher-horsepower but do not have as-efficient a midrange because of that upper-rpm balance.
Designs to modify the intake (variable-length intakes, like MV Agustas and some upper-end cars have, and of-course the VBoost itself) and the exhaust (EXUP in Yamaha sportbikes, and others' similar designs) are there to maintain high rates of speed of gas flow at smaller throttle-openings, and more gas flow at larger openings as rpm's climb. It's a balancing act.
When the premier road racing machines run by the majority of the field were single cylinder four-strokes, there was the phenomenon of 'megaphonitis,' where falling below the rpm's needed to make the cylinder charging/gas flow in the highly-tuned engines of the day, would result in incomplete combustion, and much-less power. There is no additional scavenging to be-had in a single cylinder machine because of a lack of additional cylinders' power strokes. Some manufacturers went to multiple cylinder engines to make power a different way, more parts, more complexity, less reciprocating mass per cylinder, less 'undersquare' (actually, 'oversquare') bore/stroke ratios, more valves, OHC designs, smaller combustion chamber volumes, requiring more-and-more spark ignition advance to get the air/fuel mixture to burn, it's all a compromise. That was the era of the Moto-Guzzi V-8, the four cylinder MV Agustas, and eventually the rise of the Japanese four-stroke four and six cylinder machines from Honda, while the others embraced the two-stroke system for cheap-plentiful horsepower, after an Eastern European rider defected to the West, taking with him the secrets of gas flow and the relative benefits of the use of pressure waves to keep two-stroke gases in the cylinder, available for combustion, instead of just flowing through the cylinder and out the exhaust port, because of the relative inefficiency of the piston-port design. The Japanese became-aware of the expansion chamber design parameters for two-stroke engines, and changed for many years the optimal roadracing engine configuration.
So, what does that all mean to you and the Cobra exhausts? If you like the style, enjoy it. Not everyone needs the optimal levels of horsepower, nor is it commonly used for street bikes. So, if you like 'em, run 'em and enjoy the style. Maybe down the road you will decide to try the more performance-oriented design, to see what you are missing. I'd say, a back-to-back ride of two bikes, identical as-much as possible, except with the difference being a set of 4-into-4 Cobras and a 4/1 properly tuned, will make you consider switching to a more performance oriented design.
I liked my stock design, I liked the quiet nature, the looks and trouble-free ownership. I also have three different 4/1's but only run one, and am happy w/what I have.