extractors first tho, they make the most difference, catback wont do shit if you cant get the exhaust gasses out of the cylinder (which the 1.5l 4g15 struggles with majorly) 4-2-1 design headers/extractors will give u a much better powerband and help out with flow which is desperatly needed
the reasoning;
4g15 head has 12 valves, 3 per cylinder, 2 intake valves, 1 exhaust valve
not having 2 exhaust valves restricts the flow of the exhaust gas after combustion keeping more fouled gas in the cylinder
which is bad as the more exhaust gasses from the last cycle take up room that is needed for fresh fuel and air to be let in to be burned
also the stock manifold is a 2-1 design linking non-synced cylinders which at high rpm makes terrible power as it can blowback exhaust gasses back into the cylinder beside it, leaveing again less room in the cylinder for fresh air and fuel therefore less power is made
convinced you to get extractors yet?
catback wont do shit without them on a 4g15