Exhaust options

[BP5GT]

JDM 2005 GT

How restrictive is the factory exhaust?

I'm hearing people say if you do only one thing to the exhaust do the downpipe and then get a tune.

Where I am there are virtually no shops selling brand name exhausts so one of the only options would be to go to an exhaust fabricator and have some pipes made up.
How big can I go without making the car sound like an idiot box.

I want to increase stock performance but not to the point you can hear me coming.
Should i leave the up pipe,centre pipe and rear mufflers stock?