The best way to get MPG/HP on a naturally aspirated engine would be allowing it to breath easier. I have no idea if anyone produces performance parts for the LR4, but the best start would be the intake and exhaust. After that without forced induction (turbo/supercharger) You can get modest gains from reprogramming the fuel map. Most manufacturers tune their cars to run a little rich to avoid detonation. Cams will also make a difference and on a vehicle with variable valve timing the change in the idle is negligible.
All of that being said I have to agree with Count Laszio, Why?
This is a very heavy vehicle with very respectable HP/torque If you're looking for performance in something that puts power to all 4 wheels check out a Mitsubishi Evo! Super easy to Mod, half the weight of the LR4, incredible on the track, but your LR4 has all the power it needs to accomplish what it was designed to do.