No point upgrading injectors unless they are reaching their maximum duty cycle. This would only happen if they are clogged up and have an extremely poor flow rate or most commonly because of the applicationof forced induction. I highly doubt your '04 Echo has poorly flowing injectors and if you were boosted you wouldn't be complaining of poor power. If you were to upgrade them then you would be overfueling the engine as it would be using the same duty cycle but the injectors would be injecting more fuel in that same amount of time. The ECU can adjust the duty cycle to a point, but not enough to properly compensate for much larger injectors. In my experience, a resonator-back 2.25" exhaust, K&N Panel filter and a lightened underdrive crank pulley made a huge difference. To expand on that further you could go CAI/Pod filter and a full axle-back exhaust with performance headers. Anything more than that and the cost starts to outweigh the benefits. Things like lightweight wheels, removing the spare tyre, not having a massive sound system... they all help too :)