If you fit larger injectors you put more fuel into the engine. If there is the air to burn this fuel then you get more power. That is it at its most basic. There are two options open for 25/45 etc owners, there is one option currently open for 2/4/600 owners.
200/400/600 owners – There are “wingy” or “dieselbob” spec injectors. These give smoke using the standard map/turbo, so if you don’t like smoke then these aren’t for you. They’ll give a huge chunk more torque and power over standard ones, but they do make smoke on a standard setup. You may also find that your clutch won’t like the extra torque in the midrange
25/45/ZR/ZS owners – The first option is to fit injectors from a 2/4/600 which will give a nice performance hike with a little smoke. The second option is “wingy” / “dieselbob” spec injectors and these give more power and more smoke. However it is easier to remap the car to remove some of the smoke on these models.