Generally speaking the higher geared options are all on the high range geartrain. I can't rule out the possibility that someone is making options for the low range gears too, but it's usually left alone.
With very high gearset the engine might labour a little at most UK road speeds. It's not quite the same thing but I was using a Roamerdrive overdrive unit for a number of years and it was fine for steady cruising at 70 on a flat motorway, but you had very little flexibility for easing in and out of the faster moving traffic in lane 3, and speed dropped going uphill. I now have a high top gear optioned R380 gearbox and no overdrive and it's not quite as high geared. It's an easier drive on motorways now as I can ease in and out of lane 3 traffic without getting in anybody's way and I can keep the needle on 70 on hills. It's revving slightly higher, but doesn't appear to be appreciably more thirsty.
I see Ashcroft's have a variety of ratio options available. I'm wondering whether to go a little higher next time I have my transfer box apart, or whether to stay where I am . . .