Geoff
It seems to depend on the age the type of re-map that you can do
I had my 52 plate facelift Disco td5 done by a guy Harry Devine recomended in Falkirk.. Don't know the exact figures are, but it certainly make a big difference to low end torque and top end performance. On a recent hard motorway/cross country run down to Driffield it returned 31mpg Tows alot better as well - pulls well with a Defender on a car transporter.
It was done by downloading the data from the ecu with a reader, and sending it somewhere over the internet, and a new file was sent right back. Reading. remapping and uploading the file took less than an hour.
Its been done about 10 months now and no overheating or manifold problems
Duncan