Definatly wouldn't go past an L67. Got one myself. Not calais though, since the Calais are a bit heavier because of all the electronic crap in them. My factory Supercharged VY with no mods by me (unsure what Holden did) kept up with a VZ SS (LS2) with CAI and new exhaust.
As for fuel usage:
If i'm not hammering it:
-> H/Way: Best i've had is 6.4L/100km
-> City: 9.3L/100km
When giving it to it (lets face it, it's Supercharged so you want to do this lots) your looking at:
-> H/Way: 11-12L/100km
-> City: 13-18L/100km
*FIGURES WERE TAKEN BEFORE MODS*
Right now my car's sitting on 13L/100km, driven mostly "city" (damn Coburg in Peak Hour) and a bit of freeway. That's running over 2 days, with a homemade CAI.