I can't personally speak for the V8, but on my 2009 3.8 V6 I "can" get 29 MPG on the highway (shows 30 MPG on my dash display, which is about 1 MPG higher than actual). I suspect that part of the discrepancy vs EPA highway numbers has to do with I am driving at 65 MPH, and my Genesis has 2 over-drive gears, while EPA tests are probably done at 55 MPH. But that is just a guess, and I don't really know why I "can" (under right circumstances) get up to 2 MPG than EPA estimates for the highway.
I know that there have been many owners with the 4.6 V8 who have claimed better than EPA estimates, but not sure about the GDI versions.
Occasionally I could get the "30" mpg (according to the OBC) in my 08 Azera. however, that was driving strictly interstate for about 50 miles with virtually no hills, only mild slopes. Once I got off the interstate that V6 gobbled up gas as badly as my V8 Genesis.