Major power improvement can't be found short of major modification, with Turbo/supercharger/Nitrous at the top of the list.
N.A. (naturally aspirated) mods can increase power, and can be done step by step. Tuning is a major factor, required once you get past the smaller mods. With Focus Power (Tom) offering free updates as you add more N.A. mods, Tuning should come early in any build.
The Engine isn't fuel limited until you get quite high in power mods (built engine/high boost) so changing injectors/rail won't gain anything and would require tuning to work (higher flow injectors).
Best way to think of the engine for power addition is as an air pump - the more air it can flow, the more power can be made. Fuel is added to match, stock system can add enough for a large increase.
Luck
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