alternator output isn't really a concern on a Boxster. If the alternator spins slower, the regulator will adjust and the output will stay the same. Only the maximum output will be affected.
The link you're posting is from a High-Output Alternator manufacturer, for guys who maxed out their stock alternator because of their big sounds system and 12 lights and 3 light bars.
The A/C system is regulated by the Thermal Expension Valve, if it's correctly charged and in good condition (no or very little moisture in the system), you shouldn't have problems.
Same for the water pump, the flow isn't regulated by the pump but by the thermostat.
A wp and thermostat in good condition should be able to do the job without problem.
On the other hand, an underdrive pulley will reduce the risks of cavitation, which can be big problem with metal impellers and will dramatically reduce the pump's efficiency.
It will also reduce the common overheating problem of the power steering pump.
Why isn't it factory? Same reason why the engine doesn't make 300hp from factory, same reason why they chose to use a sealed bearing to support the IMS. Engineers aren't perfect, they have to compromise. If they loose 5hp but can increase the life of the alternator regulator beyond the point of factory warranty, they will.
And as I said, 991 gt3's do have an underdrive pulley from factory. And the 911r a single mass flywheel. Things that you don't find on regular 991's.
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