Well, first let me say that I have never replaced a crest on a Porsche, so take my advice for what it's worth. I do however running a large Plumbing and HVAC business and have dealt with broken screws/bolts, etc for many years.
I would highly recommend just drilling them out. use a small drill bit with a good variable speed drill and a proper drill bit and start slowly. remember, don't force the drill...the biggest mistakes my apprentices make is they force tools....let the tool do the work!
You may need to hold from underneath with a vise grip,( I would recommend holding the head of the nut with a box wrench or with a ratchet) but if you drill from the top, it shouldn't matter if they spin or not, once they grab, you'll get the bolts out.
Good Luck!!
