I've got a 18d with the oval tip. I don't really like it and want to change the whole back box (or muffler as you say down under) to something else. There are a few manufactures now doing back boxes for the X1 (Hartge, Supersprint, Remus etc) but they're very expensive.
I hope that as the X1 is now filtering into the US a tuning company will start to produce something that will be a bit cheaper.
I guess I could get something fabricated by an exhaust specialist (as we seem to have a few of those in the UK) but think the finished article will look a bit rubbish. Coming from owning Subaru impreza's for the last few years I've been spoilt by the choice of performance back boxes in gleaming 304 grade stainless steel.