Depends what else is done to the car. A stock Cobra(that also has the Cobra intake) dynos around 230whp, a stock GT dynos around 190whp so you will see roughly a 40whp gain from the GT40s and a good intake. They are a great budget head but don't spend more than $300-$500 to get a set on your car and that includes completely going through them, new springs/locks/retainers/valve job.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.