I highly recommend adding a small airhorn. I added one after I was cut off by a big rig, it makes a world of difference. If you add an air horn in addition to the OEM horns, you can keep the OEM horns for a quick 'beep beep' when someone is sitting at a green light.
See this thread on RennTech:
Horn schematics? - 986 Series (Boxster, Boxster S) - RennTech.org Forums