Get a 1.8L twin cam half cut with manual transmission , transplant in your car ,
it will be much better than your current 1.5L carb . Or you can look for GTI engine.
The cost will be slightly higher than 4K , but better than you dump 3K to modify
exhause,muffer,port polish , air filter etc etc ....
RM3 - 4k isn't really enough if u want "max" ... seriously.
Coz if u want a better engine u need to have more than that... but if u wanna mod ur n/a engine now .. 3-4k ... seriously i've been there, n it wont give u the max... In the end i had to fork out more money. So with ur current budget, u'll have to choose for now, its either u have Performance upgrades , or like the other sifos here said ur suspension settings and everything, that'll cost you too .. Thats why until now i'm still using my ori LMST suspension but with strut bars. And tires/rims will cost u another "bomb" ...
If you wanna change ur suspension, go for the whole suspension setup not just the springs, coz if u change ur springs, SOMEWHERE will go wrong... If its not the stock absorbers, it'll be somewhere else. So... Slowly research n mod to ur budget la... Take ur time, write down ur mods with the pricing beside n see which one u reli want from 1st till last mod. Thats what i did last time... LOL , n still doing it now for my future mods. Im such a kiasu fucker.