If your issue is a lack of power when your towing, here's another, propably cheaper option...
Take the transfer gears out of a Defender Transfer box and drop them into the Disco T/Box. Lots of people do the opposite (disco gears into Defender) to get the engine to drop the revs when motor way cruising in the Defender... I know from experience though that the Defender gearing is much better when your towing heavy loads...
If you need the high gearing also, use an overdrive on the R380.
One other thing that hasnt been mentioned, the 2.8TGV has a habit of eating clutches prematurely if you tow heavy loads a lot. Good solution though if left as standard and you dont tow HEAVY loads regularly.

