You have the file directory all wrong. The effect folder is useless, since the fighter folder has more priority. That was the fault of the Brawl developers
Since you're PAL, your directory would be SD Card/private/wii/app/RSBP/pf/fighter/sonic in which you would put in your edited FitSonic.pac (just replace the ef_sonic that's in the FitSonic.pac with your edited ef_sonic).
It depends on whether or not you have Gecko or Riivolution, as well as what type of effect it is.(For example, whether or not it's a Final Smash effect, normal combat moves effects, or other kinds of effects.)
Stop spreading misinformation.