I beleive the next size up for either michelin or conti from the 325x25 is the 345 which is a standard rear fitment for the dodge viper if im not mistaken which is definately too much tire for the 911 wide body. Running a wider tire especially michelin, you may run into rubbing issues. Currently the only 325x25x20 tire avail in the US is the Pirelli.

We usually dont like to import tires only because if they have flat spots on them and cause vibration when they are on the car it becomes a real PITA to ship them back to Germany to get new ones. They were also not cheap. Even bringing them in with another palet of parts it was atleast an additional $200 per tire and that was also with the current exchange rate at that time.