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Stock fenders you can shoehorn a 275 in there with exactly the right wheel.
Roll the fenders and you might get 285.
Break out the fenders and create a widebody, 305 no problem.

Question #2- How much tire can you actually use? Top drivers suggest 245/275 is maxed for a stock 986 motor. Drop in a 3.6L and you can also run more rubber. There is always a point of diminishing returns unless your goal is "freak of nature at a car show" vs measurable performance gains.