Upgrading fuel and ignition system?

Bigger injectors will make it run worse and chances are very good that changing the plugs and coils will cause more problems.
"Tuning" your ECU probably isn't possible either in most cases, it's really just done in drift movies.
If you really want to get the most performance out of the car put a clean air filter in it, people always forget about this simple maintenance item.
Inflate the tires to the right pressure.
Clean all the junk out of it. 100 lbs of tools, speaker boxes and sports gear make a big difference.

The injectors that came with the car easily have enough headroom to provide more fuel if it's needed, the real problem is getting more air into it.

You would have to reprogram the computer. What we call the 'gas pedal' is actually the 'air pedal'. It only controls the amount of air entering the engine. The computer uses feedback to determine how much gas to inject to obtain the correct air/fuel ratio. Larger injectors will have no effect unless you can get more air into the engine. The rest of your upgrades will add no power at all.

You would need a more aggressive cam, high output fuel pump, and a cat-back exhaust system along with the computer "tune".Your best bet for extra HP that you can feel is to just put the cat-back exhaust system on the car. Check out a Borla system.

maybe and yes you would have to do something expensive with the ECU

If its a LS motor there seams to be a far power gain to be had just by changing the tune.