Hi Jonathan
Good idea about the detector - the french police are very hot over this, and you run the risk of being stopped just to check you haven't got one in a sports car like a Porsche. If they find it (even unplugged and in the boot!) it will be confiscated and you will be fined.
Here is a useful link to detector sites (static) in France which is updated pretty often:
http://www.controleradar.org/en.html
In my experience the Rennes to Nantes autoroute is policed pretty vigorously, although due to a Government campaign radar traps are much more prolific than they used to be.
I find the french police to be pretty even handed and tolerant of sports cars provided you don't do anything silly- as the officer in a Renault V6 Megane demonstrated when he just gestured for me to slow down a little after a 180KPH dash on a deserted bit of autoroute (when he eventually caught up!) - lucky me!
I don't think they will mind too much about the PSE as a group of Gendarmes just grinned when a Fezza and a TVR howled away from the toll booths last time I was over.
The do get rather het up (quite rightly) if you speed through villages/towns so be sure to comply with speed limits, regardless of what some of the locals might do.
Beware navy blue peugeots/renaults parked up along leafy service roads adjacent to roads (radar/speed traps) and don't think you can outrun them as there is usually a police bike 'interceptor' lurking further up the road on the radio (as one brit in a pug 307 discovered near nantes!).
Enjoy your trip - the French countryside is fantastic, as are their roads and I have always found the French to be a friendly bunch especially when it comes to the Porsche.
You can pre-purchase autoroute passes, but not sure exactly how - I would suggest a look at the Michelin french web site, they may be able to give info there.
Have fun, and safe journey!!
Rob