I can speak for the mods you asked about.
4.10s are awesome! The car is always on the edge of launching hard. $400
Long tubes/off road x pipe made a huge difference in how quickly the car reacts to throttle input. $750
Both mods will require a tune, which itself will make a huge difference. $150
I also got lower control arms for a better launch, they help alot. $150
I did all that for a total cost of around $1450 and can put lengths on a stock GT in the 1/4th. And all these mods can work along with future supercharging if you do them and plan to do that. Word is N/A cams aren't so great with supercharging so if that's your plan, it could be best to hold off on those.