I gotta say I am in the skeptical camp here until I see some before and after dyno data. I have been looking at these mods since 2005 and the desnork/hi flow cone filter typically return about 1hp measured. Not enough to feel with the "butt dyno" and likely below the margin of error with a typical Mustang dyno.
Where the placebo effect comes in is by altering the intake and exhaust the car is significantly louder and "seems" a lot faster. Put it on the dyno to test it objectively... bupkis.
Subjective sensation: Louder feels a LOT faster.
Objective before and after dyno test: 1hp maybe
I believe in keeping an open mind so if you want to support a subjective "butt dyno" sensation of power, get her on a real dyno and look at the objective data.