are you sure because i think it didnt check for me but maybe it was just so expensive that nothing came up until page 15 (which i obviously didnt check)
When you complete your search, above the results grid, you will see tabs for "airlines," "flight times," and "airports." If you click on the airports tab, it will show you which airports were included in your search.
Just as an example, I tried searching MCO-ORD+300 and the alternate airports were CVG, DTW, MKE, STL. That means that they didn't even search MDW as an option for Chicago. I did the same search with a 150 mile variance and the returned airports were MSN, MKE, MDW, GRR. The most surprising thing to me, in this little random experiment, was that GRR provided the second-cheapest price (after MKE, which I suspected all along would be cheapest) - and doesn't show up in the 300 mile search.
So, I guess the lesson here is that if you are just searching for a random destination, it is not enough to just search +300, but you should maybe do a search at 200, 150, and 100?