the synergy II unit from roverron is reputed to be the best available, and releases extra power from the td4, which your profile says you have. Simply plugs into the exisiting wiring loom, and unplugs just as easily when going to the dealers (if you do) Also, replacing the maf helps in conjunction with the synergy II. Finally, an EGR bypass will also increase the general driveability.
You will never get blistering performance to set the road alight, but if that is what you wanted, a diesel landrover int the way to go.
good luck.