A highline engine is no different to any other OBD2 engine (unless its 1 of the cross over Highlines on an N plate etc) Mines an 2.8 OBD2 - engine mods are air filter, exhaust, chip, de-cat but I think you'll find that even with the mods its not far off standard mpg figures. VR6's arent really made for town driving, more of a cruiser.