That's not true. Case in point... my car with an edelbrock intake, shorty headers, offroad x pipe, pulleys, flowmasters, homemade CAI, bumper timing, FPR, etc went from running 15.2 at 90mph to 13.83 at 98.55mph with those mods. 99% of the cars here with VERY similar mods dyno in the neighborhood of 230-240RWHP. My car, on a mustang chassis dyno put down a WHOPPING 188rwhp
Now, its obvious from the track times and what 99% of the people dyno with similar mods that I was no where near 188rwhp... that's what our cars will dyno stock. Now, the dyno pull was made in 3rd gear which probably didn't help much as its not a 1:1 relationship, but still ~230-240 compared to 188 is more than 1-2%. I questioned why they were dynoing in 3rd and they said that was the way it was supposed to be done and would NOT dyno my car in 4th gear, needless to say I never went back there again.