Beyond the piggyback and header, any other power adder (aside from a turbocharger or supercharger) is incredibly marginal, so I'd stop there.
(well, who am I kidding, I would never stop...
)
Beyond that, some good aero will help on a car the size and shape of a Vitara... as well as better lubricants. Small Suzuki 4x4s are typically crap on gas given the engine size, because the drivetrains are so heavy duty.