I think it may be hard on your boxster to show diminished value. Unless the car is returned with a salvage title, how would one know the accident? Carfax doesn't always seem so accurate if a new buyer was looking for your car, and if its the car of your dream, why would you sell it
Also, you don't mention if your car is of an unusual color/options combo. Diminished value on a "rare" car is worth fighting for. Not sure a Boxster can honestly be considered rare. Nice, sure, but rare? not likely.
Guess another way is, well, figure out the actual book value in its "before" condition and run a poll here asking what one would pay for it "after" its fixed.