I reckon this would all depend on which bumper you have, if you have the S3 bumper then the cutouts for the exhaust will be quite wide and a single tailpipe on each side won't look right, if you have a standard A3 bumper and trim the cutout holes yourself (as the original bumper doesn't have any cutout at all due to the discreet tailpipes) I reckon you could pull it off and make it look good.
Somebody with Photoshop skills would be able to mock a few pics up to give you a better idea, here's how the standard S3 Cutout looks like on my car (A3):
Now imagine fitting a single round tailpipe in that large area aof the cutout and you'll get what I mean about the S3 bumper (for reference, they are twin 76mm 'Jet' tailpipes)