There were only a few model years where both power figures were given in the literature.
Rather than percentages, perhaps just "raw numbers" themselves can be best. Which would be in the 20horsepower range for a dual exhaust addition. One key thing is that this figure seemed to work best when the basic single exhaust system had smallish mufflers and piping. When Chrysler went to their more-generous pipe diameters in about 1972 and later, the "sewer pipe single" systems they had, when doubled, made for a pretty decent system, as to flow. At that time, they were seeming to focus on "flow" as a means to maintain good power output.
In the 1955 era, a "Power Pak" V-8 added dual exhaust and a 4bbl carb to the mix, for another 20 rated horsepower on a 300CID V-8. Consider that with the small piping used back then, it might well take TWO pipes to get close to the flow of the '72 Chrysler "sewer pipe single" exhaust system. 4bbls from back then were doing good to make 450cfm, too.
In reality, I suspect that only some with the most-well-calibrated "sensors" can really feel an extra 20 horsepower. What CAN happen is how much happier the engine SOUNDS at WOT and reacts to part-throttle input at highway speeds. NOT specifically air induction sounds or "sounds" of allegedly better aftermarket mufflers, just how happy the engine sounds. By observation, a "happy engine" will work the best and easiest in the particular rpm ranges of normal driving. AND increase driving enjoyment!
Enjoy!
CBODY67