Hi,
Based on exhaust systems I've used so far (on Evo and other cars), this is what I can share with you:-
i) HKS Hi-Silent Power - Nice bassy sound at high revs.Can't hear much on start. Suitable for
daily use. Moderate power gain.
ii) Apexi N1 - Slightly louder than HKS Hi-Silent, bassy low revs and gradually increases with rev. Stopped by police once, they asked to ram the gas, I did, they let me go. Was running on stock cat at that moment. Not sure if any shops still carry these nowadays.
iii) Monster Ti - Like the name implies, noisy all the way, at static and at all speeds. Not recommended for daily use.