I put leds in my parking lights in the tail light assembly but had to take the bulb apart and reverse the wiring inside the bulb to make it work.
The OEM boxster tail light assembly put the all ground (negative) on the bottom of all the bulbs and power (positive) on to the side of the bulbs. Led bulbs are polarized and assembled with pos on base and neg on the side. So Led bulbs will not just plug in and work in our OEM boxster housing, with the exception of the wedge type Led bulbs, that you can just turn around. So our OEM boxster housing is the limiting factor in using LED bulbs in our tail lights.
Thought about replacing the 3rd brake lights with wedge type of Leds next, requires 8 or 9 of them.
These bulbs below are suppose to be close to filament bulb type, but are very pricey to me. But won't be a direct replacement for our OEM tail light housing.
http://www.superbrightleds.com/specs/1156-xLX5.htm
http://www.superbrightleds.com/specs/115x-xLX3.htm
Another thing about those bulb with all the extra leds, it might be too big to fit in the holes of the housing without modifying it.
Good luck, Jay