Does making a modification to a car like a Cobb lower the value of the car when you go to sell it?

Generally any modification to cars adds no value and sometimes detracts from the value of a car. The only exception is if you sell to a person that wants that particular modification. So yes I would think it will lower the value of the Focus.

I agree RARE to see a modification that does not reduce value! Exceptions are possibly... if Gale Banks performed it, or Roush engineering, McLaren! with all documentation etc, Regards