The other thing to consider is the mechanism by which the tires generate grip. It is a combination of mechanical interlocking with the road surface and chemical/van der Waals attraction. Higher pressure on the rubber can increase the mechanical interlocking if the tire is not as soft. The dispersion forces I am not sure of but I think those are maximized with a greater area in close contact with the road, irrespective of load. But I am not sure which mechanism contributes more to load sensitivty.
Then this all changes in the wet, snow, mud, etc. 
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
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