With alittle luck and picking the proper cam you can go fairly far with the stock 94-95 eec. With the stock eec your just going to at least leave some power on the table v. getting it tuned. How much??? depends...mostly stock drivability is gained along with much better tune for WOT performance with a tune.
1. Set a goal for your car.
2. Tell this goal to a performance Mustang tuner/shop.
3. They will tell you the best way to get there and how much it will cost.
4. Get 'er done!!!
When you get to the goal go back to step one and repeat because you are never fast enough.