One benefit of the 20 inch AT4Ws I found through back to back measurement is that interior db at various speeds is the same or perhaps slightly less than my factory tires on 19s.

Road manners are similar or perhaps slightly better except a bit of an inertia feel to acceleration and braking which is to be expected with the significantly higher weight of both the tire and the larger wheel together.

The 275/55R20 has a smaller diameter than the stock Defender tire. I'm running a set of Wildpeak AT3Ws on my Defender with 20" LR4 wheels in the LT265/60R20 size and they fit no problem. I'd suggest going to this size for the AT4W so you keep the larger sidewall and stock clearance.

I would be interested in seeing how that works out for you… my Load range D 275/55R20s (32.1” factory reported height) do not seem any smaller at all than my stock tires and I find they grind the inner fenders when turning if there is any build up of snow or mud at all as there is less than half an inch of clearance.

Like I said, I'm running LT265/60R20s, so I have 10 fewer mm in width and I'm running OEM wheels from an LR4, which have a different offset to the Defender wheels.

Right.. 10mm narrower but half an inch more radius with that true 33.1” tire. This is why I would be interested in seeing how that works for someone running the AT4w tire.

I suspect it would be just too tight for my off-road biased use due to pulling debris up into the wheel wells and the chance to damage things…

Debris build up is an issue, especially on longer winter trips where your wheel wells get coated in ice. Stock it's already 'tight' and increasing the radius just decreases that clearance. (and offset won't fix it, it simply exposes the exterior fender coping closer to the center of the tire - which isn't really a good situation either.). To run a bigger tire properly in those conditions a subframe lift and radiator relocation are likely the better/proper answer. *and I just bought my 24' Defender so I'm not keen to start trimming things just yet... 🤣