Fairlady Z power up

Hi, I have a Fairlady Z(similar to the 280Z but with L20E 6cylinder) with the engine overhauled and running the original fuel injection. The car seems to be under powered. The car is running really smooth from idle to the red line but does not seem to have much power. All the fuel injection and ignition components except the injectors on the engine had been swapped out with no difference in the way it runs. I am wondering about the original catilytic converter maybe restricting my power. Can I just remove it and what about the sensor from it? Can I fit 280 fuel injectors instead of the 2L ones. Also what else can I do to up the power without doing an engine transplant.