A throttle body will only make a real difference if it's allowing more airflow into the intake, like the 4.0 to the 2.5 will. If you get more air in faster, you can put more fuel in, and make more hp. If you add more air and not more fuel, you're not going to notice a huge difference.
Without doing it all at once, it's like taking a fire hose and choking it down to a garden hose. You can put all the water in you want, but the output amount will always be restricted. If you're serious about wanting to get more power, get an intake, throttle body, injectors, and header/exhaust.