Well, JD, good buddy, I just happened to log on momentarily and noticed your PM in reference to this specific thread: To begin, StrokednTN, you basically answered your own question.
In general, in the tire industry, you will find that many auto manufacturers (especially exotic car makers such as Porsche, Ferrari, Lamborghini, Bugatti, Koenigsegg, etc.) will privately commission a unique tire product and model for a new specific o.e. vehicle application from several of the well known European tire makers. Commonly, Michelin, Pirelli, and, Continental are utilized, for they have long since been chosen as o.e. tire manufacturers for the European automotive market for decades.
The N rating (as it refers to the Porsche tire model) refers only to the fact that this specific model tire by Michelin was solicited and tested with the inclusion of Porsches' new car design and specs for a particular vehicle-model application. In fact, here is a link that briefly describes this issue:
http://www.lsrpca.com/misc_web_stuff...ed%20tires.htm
And, yes, in general, tire distributors do not recommend mixing tires that do not match in several categories: Speed, Tread Pattern (model), and, Load Bearing Capacity. Therefore, they do not recommend mixing unique N-Tested Matching Sets that are slated for a specific vehicle application with other non "N" designated tires.
However, you can purchase the Porsche N-Tested tires that are o.e. for the Porsche, provided the available sizing is appropriate for your specific wheel application and vehicle. The downside to this is the fact that typically, the N-Test designated Porsche tires only come in one specific tire size for the front wheels, and, only one specific tire size for the rear wheels. BUT, all four are the Porsche "N" type. So, if you need a tire size other than the o.e. "N" designated tires, the vendor might not want to sell them to you.
Now, since you have chosen a 9.5 inch wide wheel for the front, the ideal tire width size for this specific wheel width is a 275mm. However, you can go up, or, down (i.e., 265mm) one tire width size. And, for the rear, you have properly selected an 11.0 inch wide wheel. The ideal tire width size for this specific wheel width is a 315mm. However, again, you can go up, or, down (i.e., 305mm) one tire size. BUT, do keep in mind that if you opt for the smaller size widths, you may find yourself with a "narrow" tire. Manufacturers build certain model tires with very shallow, and, rounded shoulder edges which may allow the tire to appear stretched. And, this is why I personally recommend "ideal" tire width sizes for specific wheel widths. Please feel free to PM me for any further questions and I would be more than happy to assist you.
Corvette Store
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...I'd say whatever tire you choose, go Michelin!
Yes, Rands! The traditional "N Rating" does indeed designate a spare tire. 
