I have a completely unmodified Subaru Sti 06...

Do you need to modify the exhaust? Does it mess up the car's compression ratio if you do, and then you need to retune the ecu?

I really happy with how the car goes, just doesn't have the idle rumble that the ones with the modified exhausts/turbo backs/catback do.

Modifying the exhaust seem to make the car lose trade value - correct me if I'm wrong - but just taking from my own experience when I went shopping, I looked for (and waited for) the one unmodified STi that was available!
I wouldn't even consider anything that had any kind of engine mods done to it.

Also it is an STi and goes super hard already so I'm probably answering my own question here, not worth changing the exhaust for all I know.

Only thing I might do to it is the EcuTEK retune which unleashes a few more BHPs and faster power delivery without affecting anything, anyone had this done on an unmodified STi? is it worth it?