First off~
Everyone here will crack on your car... Personally I don't look into the Kia's at all but I do know they are the sister cars of Hyundai, I own a hyundai, so it's all good...

For the most part Kia's use the engines that Hyundai makes... I really don't know what engine is in the Kia Spectra, nor do I even know the specs on the car (sorry I don't like Kia's....lol)

Anyhow, find out the engine and I can tell ya what I know...

If it's a 2.0L -- It's probably going to be a Beta I, Beta II, or a Beta II CVVT (04)

If it's a 2.7L -- It's probably a Delta engine...

If it's anything else, I dunno wtf it is

As far as performance --

Ya, ok Short Shifter is good, a c/f muffler, ok, well, the fact it is c/f doesn't really mean shit, but the fact it's a new muffler, eh, ok... w/e -- should'a just done a full exhaust system when you did the muffler...

Anyhow... Performance... basic...
Light weight wheels / performance tires are a start...
Cold Air Intake or a Short Ram Intake is nice
Exhaust is good...
Maybe get some light weight pulleys...
Get the Intake Manifold ported, same with the Throttle Body... (or buy a Aftermarket Big Bore Throttle Body and Intake Manifold, cost more though)
Headers are a plus...
a modified ECU (dunno if they make that for the kia's, hehe)
Maybe get some kind of cheap fuel managment since both kia's and hyundai's run rich -- Apexi S-AFC is nice

If your planning on a bit more you can go forced induction (based upon the engine a turbo or super charger...) and replace some engine internals... Pistons / Cams / etc... (again based upon the engine ya got)

Some really cheap crap you can do:
Ground Wires -- the stock system on kia's and hyundai's as far as grounding out the electrical... sucks... put in some ground wires, you'll notice a increase in pick up and gas mileage.

I'm sure you could make a ghetto intake outta the stock intake piping... hmm... I think thats all I can think of right now...