Hi Nasher
I have the disco gears in my 110, with a slight ECU tune. I don't need to turn the stereo up. Gets noisy over 80ish, but we are talking about a vehilce the same shape as a brick, so if you are considering a Defender I guess you don't want to go everywhere at warp speed.
It's only a cog or two in the transfer box, so also undetectable to dealers if the car is under warentee.
Easiest way to explain it is the engine will be revving as fast at 70mph as it used to at 61mph.
I've done 43,000 miles since the tune and gears ungrade and never had any problems at all.
This will raise the gearing in high range by 15%, but mine still pulls fine in high first with a 3 ton trailer as the anti-stall does all the work, even pulling away up hill it's fine.
If you get decent tyres they should not be too noisy. BFG Mud Terrains are my favorite and they don't make much noise. All terrains (BFG) are as quiet as road tyres in my opinion and I would not use another make of tyre for general use as I've never even had a puncture. (even after a three week trip in Morocco)
I can talk to passengers normally, even at 70mph, so ZZ top should be no problem.
My 110 is now 18 months old. (48,000 miles)
Rich