I fly a 4cyl 200HP Cessna Cardinal RG. It originally came with one muffler per side, with a single tailpipe. Not exactly a tuned exhaust.
I recently purchased a "powerflow" exhaust, which is simply a tuned 4-into-1 header, with very long primary tubes, tuned for the LOW RPM of aircraft engines.
The difference in performance has been significant. I have no way to measure HP, but I can measure rate of climb and service ceiling. I gained about 300 feet per min rate of climb. Also gained about 1500 feet service ceiling.
It also increased my fuel consumption. More HP requires more fuel.
Contrary to claims about increased economy, I do not see any practical flight conditions where I can achieve better fuel economy. Put another way, a given amount of HP requires a given amount of fuel. The restrictive stock exhaust simply limited the maximum output of the engine, but when the throttle is reduced, to a given output, fuel flow matches the work required.