Coolant system pressure is tied to operating temperature limits. If you never want to see 200+, then the cap can be as low as you like. If you expect to see 220-260, then a higher rating is needed.
Boiling temperature vs pressure is shown below. Note that it's pisa (absolute), which means 'zero' pressure is actually 14.7psi. Note that with a 15lbs cap, boiling point is raised to ~250F.
Running 'too high' a pressure cap can damage things like heater cores IF you bleed all the air out and IF you ever actually get to that pressure.
10-16lbs seems to be a reasonable range for most engines, but 4-10 should do well if the cooling system isn't junk.
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Mass flow dictates everything when it comes to heat transfer.
Qt = m x Cp x ΔT
m = mass flow, Cp is heat capacity, and delta-T is the temperature differential. Qt is higher if you increase any of those. Since we can't change Cp of anything (assuming away the change in density as a function of temperature), the mass flow and change in temp is the only thing we can vary. If we reduce mass flow, we need to increase the change in temp. Which means if you have a shitty water pump, or under-driven one, then the radiator fan has to work overtime and increase either the flow of air (fan speed), or the change in air temp (number of cores, effectively).
It's stupidly easy to change the fan or water pump speed (lots of pulley sizes available!) so why anyone would advocate to try and change the delta-T is beyond me. A higher flowing thermostat design will also help ensure enough coolant is moving about.
