It can be done , and it is NOT as difficult one might think.
There are several good 2 way kits , that will fit in those locations , with some mods. But the mods are not visible , as they will be behind the speakergrilles. As far as I know , there is no replacements available that will fit without any mods at all.
I know several guys in UK have done the thing , by fitting MB quart 2 way kits inside the 996 speakergrilles. I bought a POLK kit ( ex 2540 ) that did fit nicely , with some ( not much ) glueing involved. The biggest thing is the wiring & fitting & hiding the crossovers.
The door units should be replaced too , while doing that sort of improvements. But I simply wasnt brave enough to start opening the door panels , so I left the stock woofers there.
There is a "specialist" , I think he is somewhere in CA , that can do miracles with Porsches , without ruining the looks. I just can't recall now who he was.... he helped me too. With some good advice , and I did not buy anything from him ! That is good service !
Be warned though , that when you start improving , there is no end....
EDIT : check this ,
www.caraudioinnovations.com
They are really helpful ,and do know what they are doing !!