no exhaust shouldnt affect how it runs at all....it has a turbo it needs no exhaust after that

is the idle constantly low? or doesnt it rev up and down when idling?
mine used to have a crappy idle and then die if you got off the throttle quickly and i found that the BOV itself was leaking so the engine was getting lots of unmetered air. as soon as you put your hand over the outlet of the BOV it would idle fine