most of the mods that we get from japan are designed for a turbo motor. example
1)exhaust: if we keep the exhuast and add a turbo. normally on a n/a we gain like 5 hp. if you put a turbo on it afterwards, the exhaust would give us atleast 10hp at the wheels gain. so your exhuast helps alot more if you add a turbo
2)high flow cat: ones again, same deal as the exhuast. if you add a turbo you can still reuse the cat, and you will gain even more power since its now on a turbo motor.
3) underdrive pulley: i really love this one. a underdrive pulley will probrably give you around 7hp at the wheels gain on a n/a, if you put that on a turbo, you can get up to 25hp at the wheels gain!!!
you see the reason is, if you add a turbo to a car, it will increase the effectiveness of each mod that you already have by atleast 2 times. for for example. light weight pulleys, it helps the turbo spool up much faster, so you reach full boost much quicker. you will have a much flatter torque and hp curve. same with exhuast, the bigger the exhaustt he better, it has less restriction so your turbo can flow better. that is why i'm gettin a turbo for my KA. i have too much mods on a KA to just throw away for a stock SR. with all the mods i have now on my car in cluding the new iridium plugs, i will gain atleast 35Hp at the wheels on top of the gains that will be gettin at 8 psi from a t04b.
edit: your cold air intake would have to go. the only things that you ca't really transfer over from a n/a to a turbo is the intake, the headers, and thats mostly it.