I Run 285/50/20 Cooper Zeon LTZs.
They are definitely more aggressive than the factory size, and will not fit the need for desert/bolder applications, due to the still relatively low side wall compared to the 18" options. Keep in mind my LR4 is stock. I do program a lift in when going off road, so it doesn't drop when transition from trail to trail. Also the 285/50r20 will rub on the frame horns when new, and turning at full lock. It went away after 5K miles.
However I live in the Northeast; mostly forest, mud, and beach sand is what I am exposed to, with the occasional rocky trail. I have scratched and dinged my 20" wheels a few times, but IMO it just adds to the legitimacy of my LR4. She's not just a driveway queen
I have the IID2 Tool, which allows me to compensate for the oversize tires (the dealer can set the speedo offset too). However in Maine the Turnpike speed is 70MPH (Which allows for 80MPH

) Prior to the IID2 the cruise control got sets to 77, and according to two different GPS's and phone's I was going 80. So the offset is not that great. At lower speeds it almost negligible. most factory speedometers have a +/- tolerance of 5%
I hate the dreaded "dealer said" comments because the reality is, they do not want you to do anything different than factory because it COULD make it more difficult to diagnose certain things. They do not drive them daily (at least most don't), they work on them and have no time to develop an opinion as to what works in the real world versus their isolated garage lift, and limited road test loop. When I worked at a JLR dealer I did not have the appreciation I have now owning both brands exclusively for the last 10 years.