Hi pnuker,
There is a more information in post #14 here about thicker mattresses vs thinner mattresses but the essence is that if a mattress is thicker it will compress more gradually which may have some advantages for heavier people who sleep in multiple sleeping positions. It would also depend to some degree on the specifics of the layers that are chosen.
8" - 9" of latex is generally easily enough for most people to “design in” a suitable balance of pressure relief on top and support/alignment underneath unless the circumstances or individual preferences indicated otherwise.
Layering choices would make more difference than the extra couple of inches by itself (even though all else being equal they would make some difference even if many people may not feel it) but the extra couple of inches may allow for options that are more flexible for unusual circumstances or just for people who could tell the difference and preferred it.
A coolmax cover (or any cooling technology) can certainly be part of the “sleeping cool” puzzle in combination with all the many other factors in a mattress that work together (or in some cases against each other) to regulate humidity, ventilation, and temperature. Each layer that you use including the layers, types of material, and components in the mattress, the ticking/quilting that you use, the mattress protector that you use, and the sheets and bedding can either add to or detract from the sleeping temperature of a mattress. There is more about sleeping cool in post #2 here.
Phoenix