[Jeph]

Somebody please correct me if I'm wrong, but the bulb (size H7) only has one filament.

Your headlight beam adjustment may be too high (or possibly too low). To adjust that requires a tool (hex wrench) the Boxster typically comes with (but seeing as how yours is used, it may not). Do you have an owner's manual? There's a section in the manual describing the headlight adjustment process (it's one of my favorite chapters). If not, perhaps try taking it back up to the good folks at the dealership. Adjusting the headlights is easy to do, but not easy to do right.

Try this test if you get a chance:
At night, pull up a few feet (meters, eh?) from a wall or garage door. Turn your headlights on (as far clockwise as you can [2 clicks]). Then turn on your brights. Does the light brightness/intensity* change, or just where the lights are aiming? The light's pattern will look different, but the brightness/intensity* shouldn't brightness/intensity is different, maybe something is odd (unless you have mix-matched bulbs in there).

* I'm not sure if I'm using the right words for this. Basically, imagine 2 flashlights shining on a wall, one with fresh batteries, one with practically dead batteries. That would display different brightness/intesity -appearing as a different color.