You dont need to change injectors. A fast and cheap way to get more power out of your engine is just to
remap the ecu. If you still havent got enough, you should keep in mind next steps costs a lot more.
If you try to put more diesel and air into the cylinders, you will also need to upgrade the exhaust system.
A bigger tubo and larger injectors wont do much if the engine cant push out the exhaust.
Your brakes should also be upgraded if a remap isnt enough, M3 brakes maybe.