Did you read the entire post? He changed his cat to a high flow, cut a hole in the airbox, AND he also changed the entire cat back exhaust for that 13-15hp gain which was measured over a very very long time. 83k/93k baseline vs 161k modded. Thats several years to rack up 70-80k in mileage, and that final could have been a cold day in March vs a hot humid day in August for baseline, those links dont work.

Doesn't really matter though, I dont know how you can compare the gains in a rotary engine to our piston engines in this aspect. The engines are too different. Rotaries just have exhaust, and intake ports, not all the valves we have. Exhaust gases are travelling faster there

The Rotary will gain much much more from decreasing back pressure than ours will, they also spool up a turbo much faster than ours for the same reason. Also since he did the cat back exhaust and highflow cat at the same time you couldnt get much from that even if you could compare the 2 engines.