I heard a "thud" behind me...

[thstone]

1. The temp needle did hit the 250 degree mark for at least 1-2 minutes. Besides the water pump replacement, should I check anything else?

Do a quick check of the oil just in case a head gasket blew from the high temp. If the oil looks normal, you're fine. If its milky or frothy, you have a problem. Most likely the oil will look fine and the engine will be fine.

2. Do I have to use a genuine Porsche water pump or can I get an OEM substitute?

Do a search. This is been discussed quite a bit.

3. Is this a good time to install a low temp thermostat? I read somewhere that it might boost horsepower.

Yes, good time to install the lower temp thermostat. No, it won't boost horsepower (at least not in any amount that will matter) but now is also the time to install a underdrive pulley which will increase horsepower. Do a search on under drive pulley or UDP and you'll find lots of discussion.

[Meir]

Just to add on Thstone.
It will be a good opportunity to replace the front engine mounts if needed.
Also inspect the hoses next to the pump. Make sure they are not damaged by the flying pulley. Its recommend to Use only the OEM pump and coolant. (saved you some reading )