I am not familiar with the Porsche/Britax seat, but I assume that it's a "convertible" seat that can be installed rear facing (for infants) or forward facing for toddlers. If that's the case, then I suppose that the deactivation buckle would engage (to disable the passenger's side airbags) when the seat is mounted either forward or rear facing. The Seatcheck.org site recommends that all children 13 and younger should ride in the rear seat (in vehicles with a rear seat), which would also mean that they would not be exposed to a frontal airbag deployment.
I don't have much detail on the Boxster's restraint system. I would presume that by the 2000 model year they are using reduced airbag deployment force, as permitted by US regulations since around 1998. The change in the regulations was in response to reports of children injured or killed by airbag deployments. Prior US airbag regulations required that a 170-pound UNBELTED adult male must be restrained by the passive restraint system. The prior requirements led to very rapid and forceful airbag deployment, which was not ideal for children and smaller adult passengers.
I do not have the Porsche child seat airbag disabling system, but if I did have it I would disable the passenger bags when carrying a child who is under 100 pounds and/or under 4' 9" height. My opinion on that is based on the tips listed here:
http://www.seatcheck.org/tips_safety_tips.html and also based on what I know about restraint systems. My 7 and 11 year olds love to ride in my Boxster, but I have no way to disable the passenger's airbags. It's not my daily driver so they're not in the car very often...