Interesting , I also read your same question on Boxster board.
As mentioned before, some circuits are not behind a fuse, the big ones...
Anyway, have a look of the starter cabling too ? Anything broken there ? Torn rubber shielding, generally dirty or something in that nature ?
Same to the alternator.
I am no expert on Boxster, so I have no idea if anything can be seen, or visually inspected.
There is another way of testing it further, you need a proper reading current meter, so no milliamps.Your current draw is bigger, but you know that already!
Again, same principle applies, measuring AMPs, meter between battery+ and positive cable.All off, key out and so on.At this point you most likely have a reading.You said you have tested with all fuses out, still draws current, so forget the fuses then.
EDIT: I wrote a long article of how you can test it further, thus hopefully pinpointing it to the starter or to the alternator.But since they are high current devices, and not fused , I will not advice you what you could do.The risk of frying your car is too big.
If you can't see easily anything suspicious on your alternator or starter wiring, and their connecting points, take it to the Pro's.