For an undersized pulley, do you need another size belt? I am guessing we are talking a pulley that run the water pump, alternator, etc. How does that work?
Why would porsche not have done that?

Is it a DIY for a novice?

Does the undersize pulley give an improvement you can actually feel?

Yes, Yes & Yes...
Yes you need a different length belt - usually from the underdrive pulley supplier.

Yes a novice can do it if you have a new bolt, Loctite and a good torque wrench.

Yes you can feel the difference but only at low to medium engine speeds. They say about 5 HP max.

Two points of reference: A) don't buy a cheap pulley with large "open" section & thin arms - they break, and B) you lose the engine TDC timing hole drilled in the original solid pulley.

It's not, you are citing someone trying to sell you a product, try searching engineering pages. I can just about see the Brembo building from my house and have met plenty of engineers that work there and got plenty of info on this, they do not help cooling period. Slots help gasses escape although holes can too but it has nothing to do with cooling, 100% there for weight reduction on large rotors. If anything the holes create less braking but the weight offset is worth the miniscule reduction in surface area.
Ok, I'm done going off topic in this thread.

5hp, I'll pass on that idea. Doesn't seem worth the trouble. Thanks for the info.

They do. The latest GT3's have undersized pulley.

5hp for 200$ is a bargain on a naturally aspirated engine (and especially a Porsche), don't expect anything cheaper than 100$ per gained HP!

Before remapping, you'd better modify other stuff or the remap will just be a waste of money.

At the end of the day, don't expect your 3.2 to gain more than 30hp without throwing a lot of money at it (plenum+TB, exhaust, camshafts, airbox, and remap).
And of course, no more passing emission testings.

Most of the Kits come with the correctly sized belt.

By far the cheapest easiest 5 HP your going to get on this car

It was the loss of tdc on the pulley that I was worried about. How is that handled when one needs tdc for a later project?

You keep your old pulley and don't grind the boss completely.
You don't need the lock the engine at TDC every day ;)

Does the underdrive noticeably lower the output of the alternator, affect the coolness of the A/C, or water pump effeciency? Is there no downside? If not, why wouldn't factory just install an underdrive to gain a free 5hp?

I'm even seeing some not advocating the install of these things...

https://www.powerbastards.com/articles/underdrive_crank_pulley.asp

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.

Thanks, so basically, the tradeoff for a little more power is possible alternator/regulator longevity (I read somewhere that in order to compensate, the alternator will operate at more torque), but possibly save other accessories if car is tracked. I can see how it could be a good tradeoff if you want to squeeze the most out of it like the factory GT3 does as you point out. How bout their claims that the car's ECU will detect the deficiency and accordingly dial back the power output?

In my case after installing the UDP, I noticed that the water temp would start to rise a little faster if I was stuck in traffic, presumably due to the water pump turning a little more slowly? However it was not a big deal and when the high speed fans kick on it stabilizes. I could definitely feel more of a kick in the pants in 1st and 2nd gear after installing it. I also did it to reduce load/heat on the power steering pump which is known to fail due to heat during sustained hard driving on the track. Although I never had any trouble with that before I installed the UDP.

I can def see advantages to doing this if you're tracking the car.

It is true that newer cars don't like seeing low tension.
But the Boxster being a small car without a lot of accessories and electronics, it will be perfectly fine with its stock alternator, even with an under drive pulley.

If I don't sell my car, it's my next move!