So I've had a couple of Range Rovers including my wife's 2006 RRS supercharged, with 275/40-20. A little shorter sidewall than yours, we've had it on the sand but yes you can't drop air pressure that much, it's just not that great in sand. Maybe 25psi? You just don't have a lot of squish to help you in the real soft stuff. The "hot combo" for the Range Rover sport crowd for the mid2000's is a 265/50-20, that's as fat as you can go on stock rims without messing with lifts & such. So I bet you can go up one or two aspect ratios, on your stock rims without changing anything & get pretty decent sidewall.