The "reason" for increasing tyre pressures for high load/high speed has nothing at all to do with tyre wear... but everything to do with preventing tyre blowouts.
With higher loads the tyre will deform slightly more, the "bulge" at the bottom will be slightly bigger, even though the tread remains "square" to the road. As the wheel rotates the position of the "bulge" on the tyre remains at the bottom, but the tyre has rotated so to the tyre the bulge has moved and that part of the tyre no olnger "bulges" .. it "flexes" back to flat.
This constant flexing of the tyre is not constant within the tyre walls, the different "layers" have to move different amounts. Thus the layers "rub" against each other and cause heat.
Heat causes the tyre materials to soften. The tyre then "bulges" and "flexes" more, so the friction between the layers increases, the tyre gets hotter .... etc etc etc. The temperature can rise far enough that a catastrophic failure of the tyre wall occurs .. a blow out.....

High speed motoring for continuous periods has the same effect eventually, the normal "small" bulge causes a small amount of heat build up initially which then starts to increase rapidly.
This can be proved quite simply. Leave your tyres at "normal pressures" and do a quick 50 miles at high speed, stop, feel the tyres. You will be VERY surprised.
The higher air pressure supports the load, "strengthens" the tyre wall, and reduces flexing, all of which reduces the heat build up.
It is a pain adjusting the tyres everytime .. but I change mine every time I tow. Then drop them back for normal driving.
Manufacturers would not put the information about increased load/speeds in the handbooks if it was not neccessary
Not worth the risk of a tyre blowout on the motorway at 60 mph with the 'van on the back .... which could get a tad interesting ... NOT...

Just my thoughts .. nowt else
