Unless you find a way to add more air to your fuel mix, bigger injectors will only make you run rich without any gains. Maybe even a loss as the environment in the cylinder chamber becomes too "wet" from all the unburned fuel.
Unfortunately, there's not much out there for the 5sfe. You can find a header , freer flowing exhaust and a CAI and maybe pick up 5-15 HP.
A lightened flywheel and pulley set will help you get more of the power to the ground and rev up quicker.
Cams, cam gears( for degreeing),higher capacity fuel pump,larger injectors and a tuned ECU are the best ways to get HP from an N/A.
But I know of no company that makes cams for the 5sfe. And the rest of that stuff is useless without them.
Turbocharging or Supercharging would be the way to go.
You could make a 5sge out of it. Just get a 3sge with a spun rod bearing, use the head, manifolds, and all of the electronics with the 5s bottom end. Now you have a 2.2L with a freer flowing head and more agressive tune. You don't even need a BEAMS 3sge, the non-BEAMS will still be an upgrade, and dead cheap at that. Smog may be an issue though.