Just bought a V8, so, speaking as a complete novice
There are four efficiencies to consider - Volumetric (how much gas flow can you get efficiently), thermal (how much can you extract from the fuel), mechanical (how much do you lose driving all the bits, like pumping losses, large 4x4 transmission etc) and aerodynamic.
Improving volumetric efficiency is relatively easy - good gas-flow, decent cam, better exhaust, ultimately a decent turbo set-up. Thermal efficiency is harder - higher compression ratio is favourite, but the combustion chambers etc aren`t great.
Ideally the V8 needs a compression ratio of about 12:1 to make it efficient - not the 9-and-a-bit it has stock, bit it would probably detonate like mad unless you ran it on methanol.
Mechanical is much harder to solve - the transmission etc is big, heavy, has lots of twiddly spinning bits and autos are worse than manuals. You could probably improve things by fitting the standard SD1 box and making it rear wheel drive only, but probably not much and it rather defeats the object anyway. Oil levels and viscosities probably make the biggest difference.
Improving the aero is almost impossible, given the shape we`re all starting with.
Looking forward to the results, though - if it was me doing the experiments, top of the list would be a good turbo.