How you use the car makes a difference and oil change intervals should be adjusted accordingly. It is my belief that ~7.5K is the sweet spot and 15K is way too long for the filter and the oil.
In my humble opinion running oil to 15K is like running your tires to the cords - just because you can get away with it, doesn't mean you should do it.
Based on my usage (mix highway/city) and with the help of oil analysis, I've been able to determine that for my application I should to stick with 7.5-8K miles OCIs. For track usage OCIs are much more frequent, i.e., ~3K miles max.
A worth while read --->
Click Me, a real life Synthetic Oil Life Study where a group of guys took their own car and attempted to stretch the use of Mobil 1 oil to 18K miles. They used oil analysis at 1K mile intervals to evaluate oil's degradation with time and miles.
Their recommended limits: 8K mile (no make up oil added), 10K (oil topped off every ~1K) and 15K only if you are changing the filter every 5K miles. 10K+ miles without a filter change, wear goes up and the oil begs to be changed.