Jaak, just to be clear...are you saying that you know for a fact that Osram Rallye 65w bulbs will burn Boxster headlamps? Or are you saying that, in general, higher-wattage bulbs will burn? I wouldn't recommend 80w or 100w bulbs--no way--but these 65w bulbs have not harmed my wife's Passat in any way, and before that, my 1999 Miata, and both have plastic headlamp housings.
Check out these threads. I did a quick search ....
My friend has an indi P-Car shop and I see a lot of burnt headlights come in on the 996's and Boxsters. It is a comon problem. Do a search on CL or up here Kajiji sto see many of these units for sale. The lens is plastic and does not seem to take the heat too well.