Hi,
What engine are you running? If you've got either 2.5 N/A or 2.5 TD (or 2.5 Petrol) then I wouldn't bother as the engine wont be able to deliver enough power to warrent the different gear ratios that the disco T-box provides. If your running a 200tdi however then yes it is definatly worth doing, you will loose some power low down and may be changing gears more often but you'll get a much better motorway speed. I've got a disco T-box waiting to go on my 200tdi convert 90, Easter job :lol: Off-road wont make any difference because low-range is the same gearing as the 90's T-Box.
Hope that helps
Tom