Might I suggest looking into the GT Concept Rear Bumper.
It has an Air Dam and it appears to be made for a dual exhaust fitting.
I can't quite find any good pictures at the moment, but speedcorps is selling this kit and they have a few photos of it.
(IMG:
http://images.extremedimensions.com/ExtImages/08_lancergtconceptrear.jpg)
Regarding the AUS rear air dam (which I would actually like to see if anyone has a picture) I'm not sure how something like that could be fitted onto the standard bumper. I can check the service manual (if I remember) to see if it mentiones anything about this part, though I don't quite recall seeing that. Then again I have the manual for US/Canada vehicles ... i think.
Perhaps the EVO X Air dam could be fittet onto the bumper, but the EVO X bumper most likely (the entier bumper I mean) won't fit on the Lancer because of the extended fenders on the EVO.
It might just be a better idea to get an entire new bumper (such as the GT Concept). There is also a nice looking EVO X Concept rear bumper that doesn't quite exactly have the air dam like the evo, but it looks good and certainly is MADE for a dual exhaust (that's actually the one I've been looking into).
(IMG:
http://www.bodykits.com/store/images/T/B-MILA08-EVOXRB-01.jpg)