Not worth the cost unless you bump to 3:73's IMO. Like Guard said, everyone will have their opinion but in the 3 stangs I've owned, I have honestly had just about every size gear and 3:73's are personally my favorite.
Current: 2007 GTCS
*Roush> Supercharger, Suspension, Intake
*Exhaust> UPR OR, Borla Touring