I have a Discovery transfer box on my ex-army 110 with 300tdi on 750x16 tyres. Gearing is about as tall as you would want to go, but it is fine without a trailer. There must be loads of 300tdi Discoverys around on 750's with the same gearing.
I tow a horsebox with mine and hill starts can require a good boot-full of revs to get going, but you can always fall back on low-box if things are really steep. I have never had any problems with hills whilst the Landy was moving.
By comparrison, when I had the 2.5N/A diesel in it and the 1.6:1 transfer-box I found myself in 1st gear low-box when towing a Discovery on a trailer, so the current setup is a huge improvement.
As my 110 started life with an asthmatic 2.5n/a diesel it came with a 1.6:1 transfer-box, so I had to change the transfer box as it was way under geared with the 300tdi and was revving its nuts off at 55mph, by comparison it will now cruse happily at a the motorway limit. I decided to use a 1.2:1 box rather than 1.4:1 as the 1.2:1 box came with the Discovery I broke for the engine.
The TDCi Defenders have he same transfer box ratio as the Discovery, but they use the Ford Transit gear-box that comes with a megga-low first gear.
If money was no object an overdrive is the most flexible option as it keeps a lower 1st gear with the advantage of a higher 5th, although when I had an overdrive on a 4speed Range Rover I always forgot to take it out when I changed down.
If you regularly tow a heavy trailer I suspect that you wouldn't get much use out of an overdrive and you would be better sticking with your 1.4:1 transfer box, in either case I don't think the higher gearing makes much if any difference to economy, although motorway cruising is far more relaxed.