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mcrews, he isn't stating what size is better for FE and whether an increase in HP equals an increase in FE. What he is saying is that many cars come factory stock exceeding the Max HP per diameter that that chart reads. For instance, my car (with a stock 2.25" exhaust) is running about 100 hp more than the max your chart has listed (291 hp versus 185 hp). After a tune, still running stock exhaust, my car is pushing an additional 30 whp over stock.
In terms of whether the stock exhaust is ideal for either power or FE, I can almost guarantee that it is neither. It is one of the areas where the manufacturer can save a lot of money (i.e., they use cheaper, bulkier, and sometimes less-efficient materials), and they have many concerns that go beyond power and FE. For instance, they also must assure that they meet emissions testing, noise requirements, etc.
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