How much power do engine mods yield?

Johnny Dart said
... No need for dual exhaust on a slant. Just run a single 2.25-2.50 exhaust through a free flowing muffler.

A supporting "data point". I read that for the early 273 V-8, the engineers test both dual and single exhausts and found the single exhaust gave more horsepower, so that was used even in the High Performance "Commando" option. Early-A small block owners go to extremes to convert to dual exhaust (cut-out in tranny x-member) apparently for no gain.

Another. In the 1970's, I had a Honda 4 cyl motorcycle w/ 4 separate exhausts. The magazines were filled with ads for 4-into-1 exhausts, claiming "30% more power". Honda finally put 4 into 1 exhausts on their production bikes and there were then ads (from same companies) for single exhausts, claiming "30% more power". One way to honestly make such claims is to first run an engine cold on a dyno, quickly make the part swap and then run it again warmed up. Do you think Split-Fire did that?